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What's the lowest temperature and highest humidity that one can spray nitro from cans?

Tough to say for sure as you never really know what the solvent pack is inside the can. But I would say stay above 50 F for sure. The biggest problem with high RH and nitro is blush. If you have some "blush eraser" AKA retarder it really isn't that much of a problem but if you wish to avoid the issue try to keep it below 75%. Warming the spray can in a warm water bath right before spraying will help with both flow out in colder temps and keeping blush to a minimum.

I won't spray over 60% and under 60 degrees

John,
Wish I could have rules like that! My humidity is almost never less that 75%, usually higher. I get around it with retarder in everything and keeping my instruments in a 90% hot box in between spray coats. Heating the instrument surface works better for me than heating the lacquer. 40 years ago , in a cold climate, I used to put my gun in a double boiler on the wood stove to heat the lacquer. Lucky I'm still alive! Too bad Stew Mac does not sell various spray can formulations of lacquer with retarder in it to help people like the one who started this thread.- Bob

I agree using retarder does help. I use about a 10% mix what do you use Bob ?

Using Sherwin Williams and Cardinal lacquer, I mix everything 3 parts lacquer to 1 parts thinner. Then I add as little retarder as needed to the mix as I spray. Normally I add about a capful from the gallon can to a pint of the 3:1 mix. Too much retarder has caused a kind of "fisheye: effect on the finish in the past, so I use as little as possible.