Ok, let's start. don't use vent temp for ambient air temp! it means outside air temp ! you don't have to know how many oz. you have. you just put in enough that the low side is in range at 2000 rpm. never think more is better it is not . if you read a low side gauge you will note that the pounds of pressure are also the temp. the low side it. so if the low side is high. too much free-on, then the temp out the vent will be higher. So just be sure the low side is in the range of the chart its that easy! Use to high side only if you have a problem then read the high side to help pinpoint the problem.