R134a is a lighter molecule than R12, thus for a given volume of gas the weight is less. 80% of the R12 weight is a good starting point in most systems, however it can still be a little undercharged. That's where checking pressure becomes important.

Pressure should always be checked both high and low side. Comparing the two tells you what's going on with the system.

For a 90˚ ambient temp, the high side will be somewhere 200-250PSI and low side will be somewhere 35-45PSI depending on the load on the system. High head and high suction pressures indicates overcharging. High head and low suction indicates a restriction. Low head and high suction indicates a weak compressor. Low head and low suction indicates undercharging. Looking at only suction pressure presents an issue where you attempt to charge based on pressure, but can easily overcharge the entire system. Gauge pressure will vary based on the ambient temperature and the load on the system. Always check pressure with all windows open and fan on maximum speed.

It goes without saying, but never charge by pressure alone. Always weigh in the charge then check operation and tweak the charge if necessary.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)