You take the Indoor Wet Bulb and the Outdoor Temp
this link explains it - http://www.fieldpiece.com/news/article/superheat-overview
Target SH = [(3 x indoor wb) - 80 - ambient] / 2 (°F)
Usually, if you are charging an ac , the manufacturer gives you a superheat range they want the equipment to operate at (usually 6-8 degrees) The higher the wetbulb the more capacity/cooling will be diverted to removing moisture/humidity.
The super heat will show if the TXV is operating correctly as well. It is a great tool for diagnosis. If you are an HVAC dealer, your equipment manufacturer rep will be happy to explain it all to you.
All the various scenarios are to involved to train someone here.
Yikes -- Folks help me out. I am confused. As I recall, the target superheat for a fixed metering device (orifice/non-TXV) is dependent on the return plenum wet bulb (at the plenum, not in the house because what the air conditioner sees is what is in the return plenum, not necessarily the same as the wet bulb in the house) and the condenser air entering temperature.
I checked the formula given against the generic table (originally Carrier) and found the formula proposed is way off (not usable). I gets vastly the wrong answer under many scenarios. For example at 58 return plenum WB and 80 condenser air entering temperature it gives a target superheat of 34F, while the standard table gives a target of less than 5F.
Check out the linked table below from the California Title 24 Standards
On the other hand a TXV is a constant superheat valve -- so it should not change with different return plenum or outdoor conditions. For adjustable TXVs some experienced people (like Rick Chitwood) adjust it to 6 degrees F system superheat (system superheat is based on the temperature of the suction line as it enters the outdoor unit on split systems). For a TXV or EXV it is often not adjustable, but you should suspect a problem if it is high (approx15 F or above) or low (less than 3F).