The reason you not supposed to turn it upside down is that the low side port is literally the entrance to the piston. Liquids and pistons are not compatible because liquid is incompressible. You do not want to blow out your compressor rings or seals. If you let any liquid refrigerant enter the system let it expand and boil into a gaseous state on its own. You can aid this by starting up the car without the AC and letting it warm up. This raises the temperature and pressure, forces the liquid to its boiling point.
That can may have a restrictor that does not allow liquid to flow but only gas. Everything I have been taught says you are not supposed to allow liquid on the low side. If you do, just allow it time to boil and become gaseous. Do not under any circumstances allow liquid while the compressor is running.
The normal refrigerant charge as listed in the service manual is: 33 Ounces (fluid)
That figure is based R12 but R134a is a different gas and although the amounts will be close the required working volume of refrigerant will be different. Use the listing on your can to determine if you have enough. A factory charge equates to 2 1/2(Two and one half) - 12 Ounce cans(the little cans at the auto parts store).
The amount of refrigerant listed is a guide. The actual amount is where the temperature at the vent is coldest. The compressor should stay on steadily fairly steadily and not cycle on/off/on/off at intervals of less than about every 30 seconds.
