Well, here's what I do, to make sure all my designs fit in their proper cases.
1) I use a ruler, and measure the height of the plastic overlay, with the case closed. I then turn that fraction into a decimal, and multiply by 300 for a pixel count.
2) I fold or cut a piece of copy paper to a width of ten inches, and the appropriate height for the case.
3) I put the paper into the case, pushing one edge up to the glue stripe, and closing the case. This should leave the back with more room left.
4) I use a ruler to measure the gap between the end of the 10-inch wide sheet, and the other glue strip.
5) Finally, I add 10 to that number, and repeat the math from Step #1 for the final width.