You donít need a Kestrel. You need a way to measure atmospherics. This could include that particular tool.
At any rate, plug some numbers into a ballistic program for your intended round. Try different atmospheric possibilities, particularly wind, and study the solution at different ranges. This will give you an idea. A 20 mph crosswind at 500 yards can cause more drift than a slight breeze at a thousand.
Thereís too many variables to just toss out a number. This is best studied and concluded, yourself, based on your own goals and parameters. Generally speaking, however, if you want to have any sort of respectable accuracy at extended ranges, I think youíll find knowing your atmospheric conditions fairly important starting at some range shy of the 500 mark.