It depends on the scales.. their color, and how the light is hitting them in the picture. I do light guide lines (criss cross) to see where to set the scales, then usually do circles or semi-circles. It's hard to explain.. I just do it. lol

Here's an example, you can click on it to see it full size (quite big)

http://cerulean-serpent.deviantart.c...helby-14058411