the bad thing is you don't know what the high side is reading. yes low side pressures can vary widely depending on compressor speeds, and temps inside and outside. also vary widely with a clean vs. a dirty evaporator and condenser.
it's best to have everything evacuated and weigh in the exact charge according to a chart. it is usually slightly over 80% of the R12 weight.
John

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!