Use rulers or some other measuring stick (or template) is my suggestion. I do sometimes, but mostly I eyeball it, or inform the players what range is from end to end of the map in the various directions, and roughly where one should consider the range bands to change.
I recently got hold of a lot of transparencies and am in the process of trying to create some various sized range ruling circles, based on at least two different sizes of maps. I'm crap at this sort of work, but I'm giving it a go.