I've walked into an HVAC business before and actually sat down with a manager and said, "Listen, I have a question that is drivin' me and every other home inspector crazy."
"What is the range I should be looking for in A/C drop?"
ANSWER? 14-21 as a "general rule" I live in VA
BUT ...too low or too high you need to look for basic problems that could be causing it from, dirty filters, dirty coils, lack of refrigerant/ leak (Good luck with that one...even for pros to find), vents in home closed can even slow down air flow and create a greater temp drop near the coil as it is not moving as fast ... on and on.

Temp drop on a data tag? Wow,,that is a new one. I'll be looking this morning to see what I've been missing.
I know every furnace I've ever inspected has a "Temp Rise" right there on the data tag.

...This equipment in my opinion is the most complicated area of the inspection. A/C units.
One day I caught myself tryin to figure out why I was not getting more than a 10 degree drop on newer equipment. I'm thinkin' well anythings possible...thought and thought...then realized I've got the thermostat set at a point where it's being satisfied with the damn fan running.
It gave me exactly what I was asking for.
What an idiot.

I'd love to see a rquirement in real estate that states the seller must have had the HVAC system serviced in heat and ac in the last 12 months ...
OR require home inspectors to go to school for enough training to pass a test with some real teeth in it on HVAC systems.