Dont you mean the level of the highest summa +1? Otherwise its impossible to get any use out of this with anything except summae of the exact same level.
Otherwise, if you have a (art) level 10 and a level 15, the two combined becomes a level 13...
And i would rather make it =(combined quality divided by number of books)-(number of books*2), or something, otherwise any low quality summae is always totally useless because the combined quality gets based ONLY on that worst one.
Thats actually a very good compromise i think. Basically make an otherwise useless summae equal to a low level tractatus or thereabouts.
Perhaps give it a Quality rating based on its level(and/or quality?) compared to your own current level in that skill or art?
Long ago i added the variant of Greater Tractatus, which gets a bonus to quality based on the writers score in the art or skill(with this included in the limit on the number of tractatus of course) but taking twice the time to write.
Its actually a great thing because on the one hand it allows finding some much higher quality tractatus and on the other hand it makes young characters wary about wether they will want to "spend" that limit on the number of tractatus they can write now and write "crappy" books or try to wait and not get the bonus of being able to trade books as easily.
Has worked very well.
That assumes learning to be 100% linear and "building block" styled... Thats never true about any knowledge or skill that i know of.
My friend can write a computer program in C++ or C#, if i can write the exact same program in QBasic, which one of us has the greater skill score in programming?
IIRC the idea of the comprehension level is to add to quality total but if you read it with less in that skill you get a quality reduction.
So its pretty much just what you say.
Except that at least i would prefer an always positive quality, maybe bottoming out around 5(ie you will always get at least 5XP from spending a season with a relevant book, but get more the better the book compared to your skill), yeah something like that.