You're exactly right. There are too many variables to determine correct charge by reading low side only. You need to get a manifold set and monitor both sides.
Since you are wise enough to stay with R12 on an R12 system, you can use the sight glass to determine that the system is fully charged.
After the system is stabilized which will be about ten minutes at 1500 to 2000RPM with system set for max cooling, the sight glass should be clear.