I use Photoshop's "fixed size" feature for the marquee tool. I tend to scan in the same resolution, so 39mm chips have a fixed size, as do 39.5mm and 43mm. When I left click (and hold) I get the circle tool and I can move it around and line it up on top of the chip. Then I click on crop and the Save.

But I'll give your way a try. Thanks,

Jim