There are 4 ways to calculate Subcooling:
1). Liquid Line pressure converted to Saturation Temperature minus liquid line temperature at the service valve: LP(sat) - LT
2). Discharge Line pressure converted to Saturation Temperature minus liquid line temperature at the service valve: DP(sat) - LT
3). Discharge Line pressure converted to and approximate Liquid Line pressure and converted to a Saturation Temperature minus liquid line temperature at the service valve: DP - 15psi(sat) - LT
4). Condenser Coil Surface Temperature (measured in the middle third of the coil - hopefully in several places) minus liquid line temperature at the exit of the condenser coil (liquid line): CoilSatTemp - LT
As many of you know, most packaged systems use a discharge port instead of a liquid line port for high-side pressures. In many programs technicians have been instructed to subtract 15 psi from discharge pressures to approximate liquid line pressures before calculating Subcooling values. This is fine in the absence of OEM literature - or really high temps, however it may be very wrong in the case of residential packaged units, systems with relatively small coil systems, or at relatively low outdoor ambient conditions (< 80 degrees).
In the case where a TxV is part of a packaged system, the OEM will
(hopefully) provide SC goal information. In many cases the specified SC goal is a Total Subcooling reference - that is it is calculated using the actual Discharge Pressure converted to saturation temperature - not a derived saturation temperature calculated by subtracting a default PSI from the discharge pressure to approximate the liquid line pressure and saturation temperature.
Furthermore, on many packaged systems, the OEM will often provide a chart for SC Goals at different outdoor temperatures. Therefore when a tech indicates the SC goal - we would need to know if it is a single point specified by the OEM, an average derived from a chart or graph over a broad range of outdoor temperatures, or an actual value used for a specific temperature (i.e. at 95 degrees). Subcooling values and goals can and will vary by about 3 degrees over the typical outdoor temperature range, and if one is not aware of the SC goal relativity, then an "evaluator" could fail a job. This misunderstanding could also cause the contractor/technician to mischarge the job by using the wrong SC calculation or goal.
Put another way, if the OEM specifies a SC goal and does not provide a liquid line service valve, then they are most likely specifying Discharge Line Subcooling not Liquid Line Subcooling.