Here is a video on an "accumulator system" that shows how to measure subcooling of the condenser. There are other hvac industry videos I think are similar, but this video is on a car. It is nauseating however has the details of using a pressure-temperature chart on a car so I linked it here.
The superheat of the expansion valve would be similar. Except it is measured on the low side. BMW uses riser pipes with access ports, and those locations by the ports are probably not the best for taking temps, as the temps may be much higher because of the risers.
A shop can evacuate and measure the refrigerant in the system (in case of overcharge they will know). And with the system empty it is a good time to change out the expansion valve if that is the problem. The desiccant bag, on earlier BMWs are replaceable, should be changed as well when a system is opened. In case the lower pressure is caused by ingressed moisture freezing in the expansion valve.
Again, I used the ghetto method of just bring the pressure back to where I knew it was for the ambient and that worked great for me.
Example of one type of E90 desiccant bags:
https://www.rockauto.com/en/moreinfo...=16858&jsn=454