Crop guides that match proportions of journal pages

Framing scans is a little tricky because the camera view is wider than my journal pages. I think it would be helpful to have configurable crop guides to make it easier to capture all text on a page without accidentally getting some text from the opposite page.

Journals commonly use ISO paper sizes like A5 or B5. Those all use the same proportions, so that could be a good default.

It would be helpful to have some presets for other proportions, like for US letter, legal, and composition books. Maybe also an option to enter custom proportions.