If you are using a low pressure gauge when the low pressure reading is between 25 psi and 40 psi, the system is fully charged. For a high pressure gauge when the high pressure reading is 200 to 225 psi for R12 or 225 to 250 psi for 134a, the system is fully charged.
High pressure and low pressure readings vary depending on the system and the ambient temperature. Higher temperatures cause higher readings. You can always refer to the manufacturer's specifications to determine correct operating pressure and refrigerant capacity for your particular system.
When you take your readings, check the manufacturer's specifications if you want to be more exact in terms of how much refrigerant to put in the system.
