I'll first suggest you search under my user name for a thread within the past 2 months. I just did this project and provided lots of info on the topic in that thread. Furthermore, I will second what the others have said: gauges read the same pressures no matter what, and 2.2 - 2.4 times ambient temperature high side is a good way to measure. But sometimes an even more important pressure is the low side pressure (which is what produces your cool, in cabin air). You are looking for a charge that produces 25psi low side. Remember, all these pressures are read with the a/c on max at and the engine turning 1500rpm.

Good luck.

Greg
'84 300D