If it were easy, they wouldn't call it work.
Try to install a sheet first that has no cut outs. Get this sheet installed, glued, nailed, plumbed, etc. If there is no such sheet, then draw a vertical line with the help of a laser, plumb bob, or carpenter's level that represents one of the edges of your sheet. Measure from that full sheet or line to each side of the electrical rough in box (make sure you remove the finish plate). Decide whether your sheets are more important to have fit at the top or bottom. If the fit at the ceiling is most important, measure from there to find the top and bottom of the electrical rough in box.
If you are cutting this out of the sheet with a circular saw or jig saw, it would be best to make these marks on the back side of the sheet. Be careful, it can get confusing - stand the sheet up as it will install and write on the back side: top, right, etc. Lay out the measurements. Cut carefully, do not cut past the lines. I usually cut my box holes slightly undersized which may require pulling the screws completely out of the receptacles and bringing the recep out through the hole in the panel (kill the recep, tape off the side screws, or BE Careful). An electrical inspector can demand no flammables exposed to the recep. They sell a thin box extension like this:
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that can be used to meet his requirements. To use them, you will need to make your hole line up exactly with the rough in box.. ______________________________ Keep the whole world singing . . . . DanG (remove the sevens) snipped-for-privacy@7cox.net
"NetWorker"