I finally determined that my Trane has an Accutron Thermostatic Expansion Valve (TXV).
I am familiar with the superheat method of calculation for adding R22 but have doubts about using the Subcooling method which I understand some mfgrs specify.

I have a temperature pressure chart and Trane literature says to use 12 as a Superheat goal. Is it OK to use the Superheat method with a Trane (TRW036-SF-1A)(about 1993 model)? The chart outlines a Subcooling method for TXV systems. This seems similar except for where you measure the temperature. For Superheat chart says to determine saturated suction temperature from the chart, then for the Subcooling method says to determine saturated condensing temperature from the chart.

What is the difference between saturated suction temperature or saturated condensing temperature?
I believe it is the same, large pipe outside unit, suction temp as it enters outside unit.

I have calculated Superheat from the figures the last serviceman left and one system is now at 6 and the other unit at 3. The 6 was 54 before he added 4 lbs of R22. It has been working that way for 3 years, I want to check to see if it leaked R22 in that time. From my newly gained knowledge it seems that both 6 and 3 are low for Superheat which should be 12, as above. With a TXV in mind though it may be equivalent, I don't recall where he measured the temperature but think it was outside the unit on the large pipe (suction). It looks like the subcooling method wants you to measure liquid temperature entering the TXV. So I am confused about the subcooling method. The end goal is the same, 12 per Trane, but where you measure the temp is perhaps different. Thanks much.