ac evaporator

Since you are using R12 read the service manuals. The operating / test conditions they outline in there should be 'by the book' both literally and figureatively.

I would guess there just might be a slight difference in capacity. I'm not trying to be smart, that's what thermometers and pressure gauges are for.

The only time a set amount of refrigerant is relevant is when you have a known, "as built" system.

You cannot properly test a system with no effective load "in the box." With a 60* ambient I'd say that is gonna be tough