If it has no adjustment, it will overfeed a bit with R12 if its a fixed 134a valve. If it's adjustable, you will have to turn it in a bit probably. You should check superheat, which is the difference in temperature of the gas at the outlet of the evaporator(wear the sensing bulb is mounted) and the saturation temperature (measured using the low side guage pressure and 134a pt chart). Superheat on automotive is typically 8-10 deg F