you will find in the r12 system you will have to run higher pressure then the 25-30 that and r134a system is rated for. The R12 is colder at higher pressures and thats what the whole system was designed for.

Now assuming you pulled the first can in with the vacuum that was on the system, all you really need to do is satisfy the pressure switch. Once the compressor runs consistantly idle the motor up some and add a touch more if it begins to cycle again. The lower the pressure you can keep in the evaporator the cooler your air will be.

Ambiant temp here is 93F sitting and idling.