Got lucky as the underlayment was 8 ft and 1/4 inch long and from behind the toilet to under the cabinet was 8 feet an 3/8 inches.
If I can figure out a way to make the piece fit I'm sure as hell not going to cut it in half just to make it easier to install. One less seam is one less seam, especially with this crappy quarter inch "underlayment grade" plywood. I probably had a 16th of an inch gap all the way around the walls and slightly less on the seam. I make a pattern exactly the same as you would if you're installing vinyl and then I just cut it a tad less than my markings.
This one fit too perfectly so I took it back outside and trimmed a 16th of an inch off one wall by the toilet using a belt sander.
If there's something that makes a not drop into place in one piece, I'll just pattern out the entire board, then cut those areas out with a sharp jigsaw blade with no curf, and then reinstall the pieces after I drop the big piece in place. I undercut the door casings so I needed to notch out both of those and of course it wouldn't drop in place into the toilet supply faucet.area, so I notched that out too. For kicks and giggles I put a quarter inch bead of latex caulking around the perimeter walls around that toilet area before dropping the piece in place. I also put a bead around the toilet hole.
If I can figure out a way to make the piece fit I'm sure as hell not going to cut it in half just to make it easier to install. One less seam is one less seam, especially with this crappy quarter inch "underlayment grade" plywood. I probably had a 16th of an inch gap all the way around the walls and slightly less on the seam. I make a pattern exactly the same as you would if you're installing vinyl and then I just cut it a tad less than my markings.
This one fit too perfectly so I took it back outside and trimmed a 16th of an inch off one wall by the toilet using a belt sander.
If there's something that makes a not drop into place in one piece, I'll just pattern out the entire board, then cut those areas out with a sharp jigsaw blade with no curf, and then reinstall the pieces after I drop the big piece in place. I undercut the door casings so I needed to notch out both of those and of course it wouldn't drop in place into the toilet supply faucet.area, so I notched that out too. For kicks and giggles I put a quarter inch bead of latex caulking around the perimeter walls around that toilet area before dropping the piece in place. I also put a bead around the toilet hole.