Great link Rotts4u
I am not a AC tech,I don't even play one on TV
In my opinion R-134 is very sensitive to both over and under charge.
I use a thermometer to top off my system. I place a thermometer inside a AC vent that is visible out front. I add R-134 until the temp stops dropping. That is what I consider full. I have a set of gauges and an ambient temp/pressure chart I use to ball park it. I am not sure if it is the correct way, but it works for me.