In response to Fintail's question:
It is really worthwhile to pickup a manifold gauge set when servicing your own A/C.
These are often priced in the $30-40 range.
When doing a partial refill, you really can't go off weight measurements, because you do not know what is in there already.
The key value is the high side pressure at a given ambient temperature.
1986 300E 5-Speed 240k mi.