Upon further thought, I decided that the scribe, illuminator, bookbinder, and resonant bonuses don't affect the Textual Quality, only the Source Quality of that particular copy. Textual Quality can only go down due to scribal errors once the book is written.
Textual Quality = Communication + Artes Liberales
+ Ability the book is about / 3
+ Art the book is about / 5
+ 1 if author's Language score is 6
+ 1 if writing time is doubled
+ 1 if author works overtime (aging roll)
+ bonuses from author's virtues
Source Quality = Textual Quality / 2
+ 1 for each of skilled scribe, illuminator, and bookbinder
+ bonus from lowering summa level
+ resonant bonuses if arcane
Makes art books generally higher quality than ability books. It is a lot easier to get to art 30 than to art 18. Maybe you want that. I don't know.
The causal logic is sound. I am not convinced that the balancing is. I think +10% quality on a sound book is overrating both binding and illumination. OTOH, I favour the simpler core rules anyway, and the target audience have to make the judgement calls.
The art score now factors in twice: Directly as a bonus to textual quality and potentially as a bonus for lowering the quality. I'd want to avoid having this double influence.
The bonus from Good Teacher is already halved (+3 for writing vs. +5 for teaching directly). You're halving it again - is that intentional?
How do you calculate the cap to the bonus from lowering the summa level? The rules say that the bonus cannot exceed the base quality
of the summa - what is the base quality in your case?
Overtime is not really worth it. Due to rounding, in half the cases, you can double your writing time for a bonus of +1. In the other half, you'd need to both double your writing time and work overtime for a bonus of +1; I don't see many people doing that.
The way it is written, the bonus from lowering the summa level does not influence the textual quality, but the quality of a particular copy? I see why you don't want to halve the bonus, but why not decide how much weight you want to give to the single factors instead of just dividing textual quality by two (for which I don't see a good reason - why is the production quality twice as important as the content)?
Thanks for the feedback! I also talked to one of my players about it, and made some adjustments to make the weights make more sense. SQs are now equivalent to RAW once the author has an AL score of 5, I removed the double-counting of ability and art scores for summa quality, and doubling time or working overtime is now more worthwhile.
Text Quality = Communication
+ Artes Liberales / 2 round up
+ 1 if author's Language is 6
+ 1 writing time doubled
+ 1 author works overtime
+ bonuses from author's virtues
+ bonus from lowering summa level
Source Quality = Text Quality
+ 1 for each of skilled scribe, illuminator, bookbinder
+ resonant bonuses
As an example, a young maga with Com +1, AL 2, and Good Teacher could still write three tractatus on her specialty of Intellego off the bat with a source quality of 8 as opposed to the RAW 10 (with no resonant materials applied to either), but she would have plenty of room for improvement.
RAW: SQ = 6 + Com 1 + Good Teacher 3 = 10
House Rule: TQ=Com 1 + AL 2 / 2 + Good Teacher 3 = 5, SQ = 5 + 3 scribe/illum/binder = 8.
I'm not sure the 'works overtime' bonus makes sense; working excessively hard in the lab creates a penalty -- and the 'more labor' is already accounted for by 'increased time' bonus. A different bonus different might work better in that spot; maybe a '+1 if author has Teaching of 6+' or something.
That seems very similar to what I proposed. Basically, you're replacing the flat +3 bonus by AL/2 instead of Teaching/2, and adding three additional +1 boni. This means that book quality is going to be higher than it would be if you were using the rules from covenants.
My personal opinion on the additional boni:
I don't like the bonus for high language score, it will just drive up book quality.
I honestly have no idea whether the writing time bonus is balanced. Giving up a season to get a +1 quality bonus for your tractatus?
I like the feel of making an age check in exchange for a +1 bonus. The problem I see is that this is basically a free +1 bonus for young magi. It becomes more interesting for magi in their prime. It could be limited to a once in a lifetime possibility, for when a character is really investing a lot in his opus magnum.
It seems simpler to me to have a number equal to com+A+language+1 for double writing time+bonuses from virtues and lowering the summae-7, with a note that Artes Liberales has to be at least 4 and language has to be at least 5 for the book to be written in the first place.
I would also, personally, allow bonuses for exceptional craftsmanship from city and guild for the scribe, illuminator, and bookbinder,