Originally Posted by andrewwynn

When hot ambient my low side was over 35. Let me look for pic.



45/245. It was quite hot that day. I recommend getting an AC manifold so you can see high and low at the same time.

The only way to get the correct charge precisely is to evacuate and weigh how much is put in.

When I did the car in the photo I just had a thermometer in the vent got the vent temp to 39f (90 outside).

It's guesswork without knowing the high side but with just low side you can measure pressure with the car not running you'll get a relative freon quantity but you'd have to get the same reading from a working car's baseline.