The pressures will be SIGNIFICANTLY different between idle and 1500 RPM. You should let it stabilize at 1500 RPM for about five minutes. It is also best to use a HIGH VOLUME fan to simulate ram air while you are doing this. I have an old squirrel cage fan that moves LOTS of air.

There is no magic formulat to use on every car. Your best bet is to go by manufacturers charging instruction. Once converted, you are on your own and need to undercharge then add in small amounts and proceed cautiously. The high side pressure in a 134 converted system can "run away" easily.

Good luck,