A "Shady Tree" method of judging the charge in the system is to keep your hand on the suction line and the inlet line as it comes out of the firewall from the evaporator and feel for the temperature change as you charge it.

Have the fan on high, temp set to cool, outside air. As you add refrigerant, you will start to notice the larger suction line getting colder. Once it gets as cold as the inlet line, stop charging. If you have a temperature probe, you want the outlet line temp a couple degrees colder than the inlet line temp. This is showing you have a full charge. This method works well if you have a system that uses an orifice tube in the liquid line from the condenser.

If you have evaporator valve operated system, you have to check the line temperature as it comes out of the valve, usually right at the evaporator coil, which may not be easily accessable.

A simpler way is to simply feel the suction line as close to the firewall as possible and stop adding refrigerant immediately once it feels icy cold.