What high andl ow side pressure readings seem to work well for a stock 68 a/s system that has been modified with a POA valve? I am getting 20 on the low side and 275 on the high side but cooling very well. Vent temp is 48 degrees. Ambient temperature was 85 degrees, had a fan in front of the condensor.

that fan may of screwed up your readings. just have car idling with a/c on. should have 30-35 psi on low side. and it drop 30 degrees from outside temp.

Idle up to 1500 rpm and see what system pressures are. Need more cooling on condenser. As said, evap pressure around 30-35 PSI, add alittle more gas.

The readings stated are at 2000rpm. I am concerned about excessive high side pressure if I add more gas.

I thought 134 is more like r22 psi #s 55-60 on low and 250 high. 30 on the low was the r12 for sure. But this is a conversion so I dont know if that is any different.

Nice article from this guy -
http://www.aircondition.com/wwwboard/current/44954.html

I would be concerned with the high pressure - if cooling air is good, then could it be a partial blockage, maybe this is why the suction/evep pressure is low.