Printing paper is too expensive to waste so I use the smallest paper that will allow me to print the image that I want. I never print small images on large sheets of paper and I use the smallest margins that will allow the easel to hold the paper down properly. It sounds like you are worrying too much about margin sizes. Unless you aren't going to mat the print, margins won't show anyway. I choose the margin sizes when I mat the print. But even if you don't mat the print, you can always trim print margins anyway. I only rarely mat a small print in a large mat but it can be effective sometimes. But when I do that, the print itself is small and has small margins. I see no reason to waste paper using large margins when they won't be seen anyway.
There are two main methods of matting. One is to have the window (top) mat cover the edges of the print. The other (preferred by Ansel Adams) is to have the entire edge of the print showing under the mat. But, in either case, the print's margin isn't showing. In the second method, it is trimmed off anyway.
I think of print margins as necessary waste. It's necessary simply because the easel has to hold paper in place. But I can think of no reason for making small prints on large sheets of paper resulting in very large margins unless you are going to display the print with no matting and, even then, you can trim unevenness of margin sizes.
I let the image tell me what size it should be printed and how it should be matted and displayed. Margins are not a concern to me. Also, I would avoid making your choice of image size and margin size a mathematical exercise. And don't hesitate to crop the image if that will improve it. There is no law saying you have to use the entire negative although you do want to shoot so that you use as much of the film as possible for quality purposes. (No use wasting film either.) But trying to impose an aspect ratio onto a scene just because you are shooting in a particular format is unwise, IMO. I try to shoot so cropping, if necessary, is minimized but if, for example, I have a 4x5" or 8x10" negative that would look much better as a panorama, I don't hesitate to crop.
Now, if you are displaying un-matted prints, the margins do have to be considered. Just be sure that your smallest margin size is enough for your needs. Then you can trim the larger margins any way you like.
Best