I'm teaching a calligraphy class this summer in Bozeman. The subject: bookhand. I didn't want to teach the standard Johnstonian bookhand, which seems dated and, well, English. So I'm teaching a pared-down bookhand which uses the same proportions of Johnston's foundational hand and Humanist bookhand, but has no serifs to speak of.
I've assigned homework, but of course I must do the homework myself so I can talk about the problems and pitfalls of the hand as well as the problems and pitfalls of practicing. I'm learning / remembering.
Calculating a 4 pen-widths x-height with a #3 nib ... that comes to 6mm (4 x 1.5mm manufacturing spec). But my nib must have been customized as some point, because 4 pen-widths actually came to 5mm. So that's what I used.