If you have an range with known distances marked in yards, or meters for the rest of the world, then you can figure it out. I have a Leupold that is 18" between posts at 100 yards at 3x. It is 6" between posts at 100 yards at 9x. Just draw some 1" grid on a piece of paper.
I bought the Bushnell Legend 5-15 Mil Dot but couldn't find anything about which power to range with. Some are at max power and some are in the middle. I had to break out the 1" graph paper for that one. Turns out to be 15x by the way.