I hope I'm not the last one to this party but I had a genius idea this afternoon that I wanted to share. Measuring old prints for mounting is always a bit of a challenge—pretty much nothing old is perfectly square. For books, I use a jig we made with a built-in ruler. For prints, I've always done a LOT of measuring from different angles. The problem is that the image could be 300 mm wide on the bottom and 300 mm wide on the top but not square, so it doesn't actually fit in an aperature you cut to be 300 mm.
Ta DA. (Excuse the poor quality photo, the shades are closed because it's so hot out and I can't handle the heat that would come in if I opened up for better light.)
- I cut two L-shaped pieces of waste board (after first squaring off the outer edges to be sure everything would be accurate). I cut a bevel just in case that turned out to be visually important in gauging the margin.
- Print down on work surface (this also has a piece of blotter behind it to keep it clean, because I thought I'd need the grid of the cutting mat, but it turns out I didn't, so you can just do this on a clean table surface.)
- One L on top (it's the bottom/left corner one here), setting the margin on the two relevant sides of the print as you like.
- Triangle rule next to the first L
- Second L on top, setting the margin right again, and against the triangle. The triangle keeps the two Ls square to each other as you slide them together and apart until the hole is the right size. Just keep moving the triangle as you move the guides so it is against both Ls.
- Measure once. Cut once.
I did a few and double-checked that I got the same measurements on the bottom and top, and I put the triangle inside the aperture (in the corners created by the two separate Ls, top left and bottom right here) to double-check the accuracy of everything, but after that I didn't bother.
The next step to making this super easy would be to attach thin rulers to the edges of one of the Ls so you don't have to faff around with holding a ruler in place to measure. At the moment that would take more time than it would save so I'll let you know if I find the motivation.