Pressure can vary quite a bit depending on ambient temp and engine RPM. You will need to calculate using a pressure/temp chart; do a search and you will find one either here or elsewhere on the Internet. The best way to charge is to evacuate completely and charge by weight. Do a search here - a capacity chart was posted.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280