Charging with R134a
The system needs to be charged with a PT(press/temp) chart and a guage manifold.
According to the PT chart; the discharge air temp and R134a suction pressure should be: 40*/34.1psi, 45*/40.1psi, 50*/45.5psi, and 55*/51.3psi. The air temp at the condensor and discharge pressure should be: 90*/104.4psi, 95*/114.1psi, and 100*/124.3psi.
If you have an older system that has been restrofitted the watch the sight glass until the bubbles are almost gone and then add just a little more (good rule of thumb if you dont have a gauge manifold).
Hope this helps.