This directly implies that Teaching would also affect the writing of summae, and if anyone has converted the ArM4 rules for Liber Quaestionum, them too. If you change one calculation to include Teaching, you have to change them all.
Teaching is the skill of giving instruction directly between the teacher and the student. Teaching is different than organizing ideas and writing them into a book in a way that will be useful for others. If Teaching will be used at all, I would think it would only be Teaching / 2.
In ArM4, the original Quality for Tractatus was Communication + Scribe, which was your ability to convey ideas in general plus your writing ability. This was a very reasonable calculation as far as I was concerned. In ArM5, the Scribe skill was subsumed as a lesser portion of Artes Liberales.
ArM5 simplified the Quality calculation for Tractatus to Communication + 6. Without virtues, the Quality range is 3-9.
A Quality calculation of Communication + Teaching + 3 without virtues would change the range to 0-16+; with the maximum listed here assuming a Communication of +3 and a Teaching of 10, though a determined player in a long campaign could get a higher Teaching skill.
Personally, if we were going to put a skill back into the Tractatus Quality calculation, I would go with:
Communication + Artes Liberales (or maybe allow the substitution of Teaching / 2)
I would not include a base value, as this already allows a much higher maximum value than the RAW calculation. The lower starting value is not compensation for the issues of a higher maximum later on.
(EDIT: Given the much larger maximums, I am not sure I would allow it all.)
This appears to assume Exposure. It got me to thinking for a few minutes.
In ArM5, p.164 col.2, it says: "Third, being forced to practice a trade or craft in an environment with immediate feedback as to how well you are doing has a Source Quality of five."
If you have a magus in your covenant with a Teaching skill higher than yours, and you allow that magus to supervise your seasons of teaching your apprentice, doesn't that mean you are doing Practice at Quality 5 instead of Exposure at Quality 2? Just wondering what others thought. Because I'm sure inventive players could easily think the same thing.