if one of your main ways of creating scribes is encouraging people to paint charters that have pre-printed text, you will inevitably end up with more experienced painters than experienced calligraphers.

Ye-ep. Ansteorran painters blow me away. Ansteorran calligraphers seem to be rarer than hen's teeth.

I think I'd do more scrolls if I had a lightbox and could use ruled lines underneath the paper, but my preferred paper is too thick to do that without one. I think if the preprinted lines were very very narrow it wouldn't be too bad a tradeoff for beginners; they often left lines visible in period.