Aircondition.com is used as a reference by all sorts of persons including professionals...
If you had run over and scoped it out you would realise they have already presented the answers to your question way better than anyone on this site can.

The Aircondition.com 'knowledge base' section can't be beat...

You must stick guages on both sides of the system and stick a thermometer into the center vent and turn on the system for at least ten minutes outside but not in the sunshine to establish anything in terms of a base line to start from.