Yeah, that sounds like a much more likely place to measure the temperature differential.

Once you've got that within an expected rule-of-thumb range, then you're down to the quantity of air that is expected to be pushed through the system at that differential, and that's down to the cubic footage of the house versus the capacity of the AC unit (with the house's insulation factor being the wildcard).

I just remember that I had a miserable time in the hot house on the hill. The A/C people looked at it and said that the device was working correctly and that it was the correct size for the house. The problem was that it was the worst-insulated house on the planet, with no shade cover, on a hilltop, so it baked in the sun for the whole summer.