How should I apply ambient air temperature ampacity correction factors when exact temperature is unknown?
Size it for the highest expected temps in the area (geographically and part of building - huge differences there).  If  on the border line, then size to the conservative side and use bigger conductors or more runs.

In my area, the high temps would be 105'F If we add another 50'F for an attic (worst case per your numbers) that would place me in the 155'F+ bracket.  However, a crawl space would likely just be based on the 105'F or less.