Always, i repeat always do your scans with perfectly square demensions. You can set and lock your cropping. You can also make a couple of little marks on your scanner bed so you know exactly where to place the chips, and then instead of using preview every time...you can use zoom. From the zoomed image you can adjust the crop sqaure so it is perfectly centered.

Also...your image is best at 100%(not reduced at all). I do all of mine at 200 dpi. You can do higher, but should not go lower.