I think in Bickham's UP or in the other, smaller book (Clerk's assistant? I'm not in my studio so can't look in my library) it says "cut the nib to the desired width of the stems of the letters you wish to write" or something similar (Roundhand was written with a square cut quill back then) … so I think, originally there were no absolute rules. You looked at the examples and tried to match them (or change them, if you didn't like them). Also I think, a measurent like "make the width of the stem one eighth of the hight of your letters" wouldn't be very helpful. In the end, I'm afraid, wether you prefer slimmer or fatter letters, it comes down to learn to judge their appearence by your eyes … making aesthetical judgements, not mathematical ones …
You could just measure up one of the historical exemplars, one that you like preferably, if you like exact rules
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