I have done a lot of them and although the general rule is to use about 80%, I have had much better luck using the pressures.

Make sure you use the correct oil. The R12 in the old Mineral oil systems will not circulate in R-134a and will that will cause compressor failures. It will not hurt if it is in the system AND you use the correct oil, but it won't circulate to lubricate the system. Also PAG oil is not compatible with mineral oil. If there is any mineral oil in the system use ester oil not PAG. I use ester only in R12 systems even if I am not retrofitting just in case.

I charge the system until the low side gets near 30 to 40 and the high side gets about 200. From there I feel the air temp and add a little if needed but do not let the high side get too high. The old systems do not have a high side cutoff and R134a pressures climb fast in hot weather.

As stated, R12 while hard to find is a better refrigerant.