That is not correct. If the system has all original components (no non original parts such as an updated condenser) the r-134a charge should be about 80 % of the R-12 charge.
I'll take your word for it. I'm just a shade-tree mechanic and I've always used the sight glass and a set of gauges to work on my AC systems. I'm always willing to learn new info, so thanks for educating me.