I print about the same size as you and mat my prints at home. What I do is calculate the window opening 1cm (10mm ) bigger than the actual image, and then, add half a cm. in the bottom to add my initials just below the image in the right side.
Using centimeters or milimiters lets you be more precise and the logan mat cuter I use has both scales, and you save a lot of money cutting your own windows.