Accurate tests should also be made, to determine whether proper airflow CFM is being delivered, as well as into each room.

The manufacturers could be of great help in this respect, if they would list the Delta-T of the condensing unit at different BTUH load output levels, also at various outdoor ambient temperatures.

I believe that with adequate test data feedback the subcooling could be targeted within plus or minus 1 or 2 degrees Fahrenheit, which would be a two to four degree differential.

That would be more accurate & concise a temperature target than present subcooling temperature targeting methodologies.

Additionally, I would consider using the Lennox Approach Method to help select the Subcooling Method.

The Lennox Approach Method subtracts the Outdoor Ambient Temperature from the Liquid Line Temperature (LLT), whereas, the subcooling temperature targeting method subtracts the Liquid Line Temperature near the evaporator from the Condenser Saturation Temperature (CST). I do not see why the Lennox Approach Method would not help pinpoint the subcooling target on other systems. The Indoor Heatload has to be part of the equation; there are other factors to incorporate as well.

Possible Diagnosis using Super-Heat and Sub-Cooling:
If Superheat is high and Sub-Cooling is low:
Charge must be adjusted. System is Undercharged.

If superheat is low and sub-cooling is high: 
Charge must be adjusted. System is Overcharged.

If Superheat is very high and Sub-Cooling is a little high: 
Could have blockage in coil, TXV strainer screen - settings, etc., orifice, filter dryers etc.

If Super-Heat is low and Sub-Cooling is low:  
Piston orifice could be too big, or some, in backwards, there is no orifice in the unit or the orifice is stuck and refrigerant is bypassing it.

To Determine Delta T  (Td) (Temperature difference across the coil):
1.  While unit is running take the temperature of the air in the supply plenum near the coil (approx. 12 inches.)
2.  Then, while the unit is still running, take the temperature of the air in the return plenum near the unit.
3.  Then take the difference between the above readings.
4.  Should be around 15-18 degrees.  Use linked Chart above!
5.  If to low then coil might not be seated in pan correctly - air bypassing cooling coil. (Assuming superheat and Sub-Cooling are OK.)
6. A TXV's normal Superheat setting is 12-F, between 7-F to 9-F on heat pumps with an accumulator.  There must be a full liquid stream to the TXV!

With a TXV metering device if Superheat is too high say,  20-F or above — look for, suction line restriction, plugged cap tube/orifice./liquid line, hot gas discharge line restriction, filter dyer, downstream of suction service port, or compressor screen restriction or inefficient compressor.
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What are the proper methods to determine operating superheat, sub-cooling?

Superheat at the evaporator should be checked as close to the end of the coil as possible (preferably near the expansion valve thermal bulb). Convert this to saturation temperature and compare it to the actual temperature obtained near the thermal bulb. Take the suction pressure at the service valve and convert it to saturation temperature. Compare this to the actual temperature obtained approximately six inches out on the suction line.

Subcooling should be checked as close to the condenser as possible & then as close to the TXV as possible noting the difference.
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With a TXV metering device if Superheat is too high say,  20-F or above — look for, suction line restriction, plugged cap tube/orifice./liquid line, hot gas discharge line restriction, filter dyer, downstream of suction service port, or compressor screen restriction or inefficient compressor.
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What are the proper methods to determine operating superheat, sub-cooling?

Superheat at the evaporator should be checked as close to the end of the coil as possible (preferably near the expansion valve thermal bulb). Convert this to saturation temperature and compare it to the actual temperature obtained near the thermal bulb. Take the suction pressure at the service valve and convert it to saturation temperature. Compare this to the actual temperature obtained approximately six inches out on the suction line.

Subcooling should be checked as close to the condenser as possible & then as close to the TXV as possible noting the difference.
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Determining which metering device TXV or Fixed Orifice the system has without physically looking

If you do not absolutely know whether the metering device is a TXV, or a fixed orifice device or cap tube. 

Hook up your manifold gauges, block off considerable condenser air intake for a short time.
If suction pressure changes it's a fixed piston or cap tube
If only the high-side goes up, you have a TXV.

Have things with you in your van or truck to block-off the condenser air for a short time. 
Check every time you are not certain what metering device it has. 
There may be a lot of guessing in the future.

Do this procedure on known metering devices to observe the difference. 
Report back to me how well it works for you. 

In some situations, that could save you from cutting a hole in the plenum. 

Squirrel cage wheels with forward curved blades on residential systems
unload when discharge air is blocked off too much & will overload
when there is no static pressure.
 
There is a preferable ESP range for each Air Handler blower design, that ought to be listed on the blower; they vary at the point of serious unloading.
 If you amp-probe check enough of those blower motors, if the amp draw is too low according to its rating, you can begin to tell that the External Static Pressures (ESP) is too high.
Additionally, mfg'ers could list the amp draw at various design ESP numbers, then we could amp-probe & know if it was too far above the amp rating, a duct maybe off, 
if amp reading is too low, it is time to check all static pressures & delivered CFM to each room.

I lot of us used to set a nearly empty R-22 cylinder on top of a condenser to warm it a little. Back then fan motors had more HP
& higher amp draws, therefore it didn't seem to cause any harm, just more noise.

Back in the 1960's & 1970's there were a fair number of TXV metering devices & some table top condensers' that had the fan underneath blowing up through the coils. 
Well, where there were cottonwood trees, nearby clothes dryer lint vents, or a lot of leaves or other debris under the unit, the fan motors would be blocked overload & burnout. 

I don't understand the engineering genius of that moronic design. 

However, on hot days & a heat-loaded E-Coil, 
You could move your wrist over the condenser from outlet up to inlet, & tell if the liquid was taking up too much area of the coils; an overcharged system. - udarrell

Gurgling sounds at TEV: Low evaporator heat-loads lead to reduced liquid line mass and increased evaporator mass could be due to airflow problems. Eliminate low evaporator heat-loads before looking into adjusting the refrigerant charge.  Gurgling - pulsation noises in Liquid Line at the expansion device can be caused by low evaporator circuit heat-loads, low charge, and/or non-condensibles and moisture in the system. Unbalanced airflow through the various distributor circuits of the evaporator coil will cause the TEV to close down refrigerant flow starving the coil. Piston-flow-rators will make it impossible to properly charge the system and cooling will be greatly compromised unless you eliminate the cause! "Put your ear on the liquid line at the evaporator coil."

On every Rheem condenser cover it lists "non-condensibles and or moisture" as causes for a gurgling or pulsating noise at the expansion device. The entire evaporator circuits, may not become active for various reasons, - "the entire coil must become fully active for efficient performance."

The purpose of these recommendations is to provide liquid refrigerant at the expansion device and provide efficient operation. Hopefully, this will aid your research.  If I can be of additional assistance, contact me.
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Check Return Air (RA) at grille & at entry of blower for heat gain, due to hot Return Air leaks.
Where airhandlers' set over Return Air Chambers check for air leaks through the sheet rock & down the wall studs from  the attic - this is a fairly common condition that will overload the AC system!

DISCLAIMER:
Any of the HVAC companies I list on any of my web pages have nothing to do with the information I post on any of my Web pages nor do I assume any responsibility for how anyone uses that information.
All HVAC/R work should always be done by a licensed Contractor! This information is only placed on these pages for your understanding & communication with contractors & techs.

This information is for the edification of contractors and techs. I am NOT liable for your screw-ups, you are liable for what you do! - Darrell Udelhoven (U-dl-hoven)

Darrell's Refrigeration Heating and Air Conditioning - Federal Refrigerant Licensed - Retired HVAC Contractor