Measuring For Your Mount
First, look at the work and ask yourself, "is it all really necessary?", do I need that piece where the paint fades away? (When signing your work, do not put it right to the edge, take it in an inch or so.)
Check that the paper is square, especially if you tore a large sheet into smaller pieces. Measure widths at the top and bottom of the sheet, and the height to the left and right of the sheet. Do not assume that the pad maker's size is correct; 16" x 20", as an example, may be the metric 40cm x 50cm (153⁄4" x 195⁄8"). Or, 7" x 10" may be what it says on the front; yes, but it did not mention the perforation!
Do not ask for the aperture to be cut too near to the edge of your work; it can get very fiddly to make the work fit. The aperture ideally should be about 1⁄4" (6mm) smaller than the work in both directions; that gives you 1⁄8" (3mm) to play with. Allow more (smaller aperture) for larger items.
In the past, we have messed things up for all of the above reasons, and hundreds more. Of course it was never my fault. -- Mike Dymond