The procedure we use for topping off a R-134a system that may be a bit low is to attach the gauge set, start the engine and then make note of the outside temperature. Then reference the temperature vs pressure chart to see what the expected high and low normal pressures should be and add refrigerant to get the system within these ranges. As a final test we measure the duct outlet temperatures to determine if the A/C is functioning correctly.
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "