I checked the freon in my car today because the the a/c is blowing basically whats outside, the gauge says i'm fresh out. I understand everything I have to do but i'm not sure how to check how much I need to put in. I guess I could take it off a bunch of times and test it with the gauge until it reads 'full' but that sounds like a pain. Do I just fill it up and test with a thermometer in the interior vent until it's 30-40 degrees colder then the ambient temp. outside? I just worry about overfilling it or not filling it up with the correct pressure. And do I need any special kind of thermometer for the vents? How do you guys do it?