My 16x20 silver prints are mounted the same way as Dan's. Only, I cut the window 1/2 inch bigger than the image area on the top and sides, and 3/4 inch bigger on the bottom. Outside measurement of the mat board is 24x28. And Dan is right...it must be carefully done. For my own work, I do not like hiding the edge of the photograph behind the matboard.
My 8x10 platinum prints are mounted on 16x20 board, with the window cut slightly larger than the image area -- showing the black rebate around the image...perhaps 1/4 inch top and sides and 3/8 inch on the bottom. My 4x10 platinum prints are mounted the same way on 12x16 board.
I have never had anyone ask for a print that was precisely x by y inches. I would be tempted to tell them that I don't sell prints by the inch...LOL!
I window matted a retrospective show for my old photo teacher -- actually for his widow. Thirty prints or so, all old vintage prints already dry-mounted. It was a tough job...not because of cutting the windows (I have done so many that I think I have done it in my sleep), but because of the artistic decisions I had to make. I had to overlap the sides of the prints with the windows...and deciding how much to cover, etc was tough. But generally, I tried to cover no more than 1/16 inch of the image. But many were not dry-mounted square, or the prints themselves were not square. I have always felt that the edges of an image defines what is in the middle -- so deciding how much to crop out with the window was not a easy decision.
But it was a great experience...most of the images I had cut windows for over 25 years ago when I was a student, and I earned my beer money by cutting mats for students and faculty. And working with the prints so intensely, I got to have a much more personal involvement with them than just looking at them on a gallery wall.
As for the outside dimensions, I tend to work in the "standard" sizes. 8x10, 12x16, 16x20, 20x24...I deviate from this for 16x20 prints. For 16x20, I do not like the look of 22x28 boards..especially for hortizontals -- too much "extra" space on the sides (it actually works fine for verticals), so I use 24x28.
Vaughn