I will vary it some depending on the print/mat size. For 15x19 prints, the 1/2" (13mm) on the top and sides works well for me, and I usually will have 3/4 inch (19mm) on the bottom. I like the little extra weight on the bottom. For a 4x10 platinum print on a 12x16 mat, I will use closer to 4mm of space on top and sides and perhaps 8mm on the bottom. In such a case, I am showing the rebate and exposed paper (black) in that space.
The image below was window matted to just exclude any of the paper white around the print while maintaining even borders. The watercolor paper is heavy and without an emulsion, not likely to curl. I either make corners, and/or hinge tape the work to the back-board. I sign the back of the print in case it ever gets separated from the signed mat.
My carbon prints have a very thick emulsion (coated onto the emlusion side of photo paper also), so they must be handled differently.
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