refrigeration and aircon units are charged by weight of refrigrant, not pressure, which changes according to operating conditions, so you need to know the charge weight and type of gas. I have seen a small Waeco unit that took 19 grammes of R123a, which is a commonly used gas in automotive a/c systems. If your fridge has lost its gas, you will need to find why, car a/c looses its gas through the rubber hoses in the system, but fridges dont have rubber hoses, so leaks must be from somewhere else.

hope this helps

Peter