How much 134a?

Thanks all. Finally got warm enough to work with AC. These are the numbers at 75 deg ambient, 50% humidity, doors open, fan blowing on condenser, 1600 rpm. 15 psi low, 165 high, 45 deg at vent. Am I under charged? I only put in about 19 oz 134a. The low side seems low, but the vent temp seems ok. How much under charge will harm a Sanden compressor? Thanks.