As far as literal AC function I understand the outside temp is irrelevant, but, from a practical standpoint the outside has a lot to do with how the AC cools. Our black vent covers, AC shrouds, black windows etc. generate a ton of heat inside of the trailer; all of which the AC has to counter - the hotter it is the harder it has to work. At some point an x sized AC will not cool an x sized trailer simply due to the amount of heat coming into the unit and the limitations of the AC unit.....then you get a larger unit or 2nd AC
Simplified I understand but those are factors.
At 50 degrees a +/- of 5% (2.5 degrees) is good enough for me with the infrared gun. It's also good enough for my heating/cooling contractor to use when diagnosing AC problems. The gun says its optimum reading range is 18" and that's what I used when reading the AC vents in the trailer. I do also use it for reading my wheels when traveling....much more accurate than putting my hand on the wheel/tire
It's just a good way to get a quick indicator of what's going on; beats the heck out of hanging little thermometers in the vents, waiting for them to get to max cool and then trying to read them as I've seen done.