According to the German calligrapher, typographer and book designer Jan Tschichold (1902-1974), a medieval manuscript has a page proportion of 2:3, margin proportions of 1:1:2:3, and a type area which is in accord with the Golden Section (1:φ). The lower outer corner of the type area is fixed by a diagonal across the page, and also (as far as I am able to interpret from sketches which explain this principle) by a curve with a radius which is equal to the diagonal of the page and with center in lower outer corner of the opposite page.
Except for the margin proportions (they’re not quite 1:1:2:3), the pages in my copy of Field, Factories, and Workshops comply with the classic page construction described by Tschichold.