You can find a lot of threads on dash AC as well as temp/pressure charts. If static pressure is below 20psi I would say you need to vac it down and test for bleed down time then weighed charge added.
Also those adjustable temp/pressure gauges on cheap R134A charge can adapters will put you over charge if you follow it, on system with TXV expansion valve. You need to go by chart , temp and proper rpms. if adding a can or two.
You can find guys that do HVAC on the side to vac and weigh a charge. You would need to make sure you have proper air flow across condenser and right rpms to set it right.
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