That's not how an AC, freon related problem is diagnosed (pressure, engine OFF) and there are no specs for that anywhere. As mentioned above, you need to have the freon weighed and an AC machine does that when it recovers the freon.
You can TRY adding some freon but if you're using a 1 lb tap-a-can (if sold in your state?), there's no really good way to know exactly how much freon you presently have or how much you're adding. If you add too much, the AC system will still not cool well and the system only holds 1.63 lbs so it's easy to overcharge that way.
Most shops (dealers, etc) only charge 1 hour plus freon and dye (if not already installed) to re-charge an AC system. AC dye won't be visable until the freon level becomes low again and if it's a slow leak, could take a season or two. If your car already has dye (sticker under hood), one can physically inspect your evap to see if there's dye present.