its easy to predict the low side pressure, but predicting the high side pressure by ambient temperature is a crapshoot, its very dependent on the generosity of the original condenser sizing, the fan type, and control strategy. We see charging charts based on ambient conditions on commercial equipment, but the chart was plotted by the OEM based on the original gas being tested in every possible temperature/humidity scenario Most of the older cars have a thermostatic expansion valve and with R-134a, I slowly add charge until I stop seeing a change in the superheat and suction pressure, that indicates its getting enough liquid to gain control. The sight glass will remain bubbling, but as the valve gains control and throttles, usually the sight glass will start to fill on its own as the interior cools down and the load decreases.