f57v

[Jorge_Stolfi]

He uses a compass (or a sharp pencil with a thin string)

I think it is almost certain that he used a compass, even if was a self-made one.  The string-and-pin method would have been more wobbly.  At first I thought that he had used templates, like upturned bowls or cups; but that idea had the same problem, and then I found the pinprick at the center.  Also, the traces of the circles are consistently very thin and wholly or partly inked, which rules out both pencil and quill.  He must have used a metal ink attachment like the one shown in the miniature linked to.

doesn't have a ruler to perfectly align the letters?

Good professional scribes would draw two vertical lines to be the left and right rails of the text, then mark equally spaced dots along those lines, then draw horizontal baseline guides through those dots.  All without ink or pencil, instead by scoring the vellum or paper with a sharp stylus -- so that those lines would be visible with slanted illumination, but not with "normal" illumination (perpendicular to the page).

AFAIK the VMS scribe did that only once, for the three lines of text at the bottom f67r2.  If he did that elsewhere, I could not see signs of it. 

Again, he did not know or care about planning his work by such things as drawing guide lines or dividing space into the required number of equal parts. On f67r2 itself, the straight lines that delimit the sectors are drawn with a ruler, yes; but opposite lines don't meet and miss the center of the diagram by several mm, and the angles between them vary by 50% or more. 

All the best, --stolfi