A method I used for my home a/c is to look at the temperature of the low side hose with a special temp sensor that can wrap around the hose. The refrigerant temp for R134a corresponding to the the low side pressure can be read from the gauge. At maximum fan speed, adjust the pressure of the low side so that the temperature reading of the low side hose (from the temp meter) is about 15 to 20 deg lower than that read from the pressure gauge. The problem with the usage of this rule on cars is that the air flow is variable as opposed to fixed in home a/c, so for lower speeds the system may appear to be overcharged. But try this anyway.

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Saumil S. Patel