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Unfortunately there really isn’t a “magic number.”
And those charts are usually wrong (for example the low side numbers are usually too high, for example, you probably won’t see 35 psi even at 80 degrees ambient).
IMO the temperature pressure charts aren’t actually that helpful as far as telling you when to stop adding refrigerant. Not totally useless, but for what you’re doing, pretty close to useless, and there’s no magic number. (The gauges are very useful as diagnostic tools, and for getting a general sense of the system condition).
The only way to know for sure that it is spot on is to vacuum it down and fill by weight.
Outside of that, IMO the best way is to use multiple factors:
1. Measure and watch output temp.
2. Look for condensation on the suction line.
3. Using a temp pressure chart as very general guidelines to make sure something isn’t way out of whack.
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