Great explanation Ian.

To simplify the application of the idea here a bit I might suggest thinking in ratio and understanding that it isn't normal to have viewing lines marked, rounding is okay.

If we know the viewing distance is about 36" (as if the print is to be behind a chair) and that the print size will be 16"x20" then if we use just the short edge in our calculation we get 16:36, simplify and round just a bit and you get roughly 1:2 as the ratio. (I used the short edge here because the long edge would be cropped to fit the standard paper with 35mm film.)

So if the short edge of the film is 24mm (the "1" side of our ratio) then the focal length should be roughly 48mm (the "2" side) to get normal angular relationships. If the short edge of the film was 4" then we'd want roughly an 8" (200mm) lens.

If we plan to print at 8x10 for the 36" viewing distance the ratio changes to 8:36, simplify and round just a bit and you get roughly 1:4 or 1:5 as the ratio. Short side 24mm x 4 = 100mm, x 5 = 125mm.

We can work backwards too. If all we have for our 4x5 camera is a 6" (150mm) lens and our ratio is 4:6 or 2:3, then a 36" viewing distance would suggest a print with a 24" short edge (break out the roll paper).