My 1994 SE took 31 ounces to go from weak, cycling A/C to full cold (ice forms on the accumulator dryer at times) so 20oz is in the ballpark for a top-off.
Strange part is that the gauge I used still says borderline low charge (interdynamics guage, sits right on the green/blue border) but the system is working well so I stopped adding.
Another note here.. Try to check/top off the freon when it'* warm outside. I found that my compressor will cycle when the temps are 55-60F but stay on full when 75-80F. As I mentioned before the lines actuallly get a coating of frost on them with the fan on low so I know that it'* got enough freon...