A/C Charge by Myself

Sorry but you are incorrect. The pressure depends on whether the cylinder or the cylinder has liquid inside, AKA "saturation" and the TEMPERATURE. You can't turn the cylinder upside down? really?

And then there's this, ??WHUT??

" The pressure coming out of the 30 lb'r is way less than the pressure you will see on the high side line, so it would just pump it back into the cylinder if you opened the valve."

I don't even know what that is supposed to mean. So I will explain. AGAIN. LISTEN CAREFULLY

YOU START with the system evacuated and SHUT OFF. Repeat. You start with the system evacuated and NOT RUNNING

You do whatever you do, charging cylinder, scale, etc, and get the target amount set in your mind. Get the manifold connected, fittings and hoses purged, etc.

Then with the cylinder CONFIGURED FOR LIQUID, you dump the MAJOR PART of the charge USING LIQUID, into the HIGH SIDE FITTING. To repeat for the FOURTH TIME you do this with the system NOT ENGAGED, SHUT OFF, NOT RUNNING. The liquid will flash clear through the condenser (repeating) the dryer, the control device, the evaporator, at which time THERE WILL BE NO LIQUID and the building VAPOR PRESSURE will start to BUILD PRESSURE on the low side gauge

After this has settled down, you will have a SAFE condition to START the system. You will have MOST of the charge in the system, the refrigerant will be VAPOR at the compressor inlet, and now all you have to do is run the system and make final adjustments to the charge.