In Ars Magica, Covenants, they claim that most "standard" summa as distributed through the Order of Hermes have a quality of 28-level.
Despite spending 6 hours trying to understand the math (with errata), I was not able to achieve 28-level for Arts or Abilities in any "common" sense.
What is the exact process for producing a summa with a quality of 28-level in a standard and reproducable way with a "normal" summer or autumn magi?
For purposes of this example, let us assume a magi writes a level 15 summae on the art of Mentem. How does she achieve a quality of 13 within her standard 120-160 year lifespan?
Given: An autumn magi with a communications of 4 (magically enhanced from 0), a score in Mentem of 21 (equivalent to an ability of 10, which takes a lifetime of learning and practice. We can say she has spent 28 seasons over her 80 years simply studying from vis or its equivalent.) Level 21 is considered to be exceptional, but not legendary. An equivalent technique would allow her to routinely cast level 40 and 50 rituals, explaining the communications of 4 (and it's commensurate warping.)
Thus, she can make, as per page 88 in Covenants and per page 165 in the core book, a summa of level 10.
The core book asserts that this summa's base quality is communications+6, or 10.
Page 88 asserts:
Author’s Communication + 3 +1 for skilled scribe +1 for skilled binder + 1 for skilled illuminator + 3 (or less) due to mystical resonance + bonuses due to Virtues + bonus for writing to level less than half of Art score.
The autumn covenant absolutely has skiled covenfolk, so she has 4+3+1*3.
The book is bound in expensive and resonant materials, resonance +1, has story-related materials incorporated in, resonance +1, and has been opened to enchantment, allowing for a 3 year clarification process for a total of +3.
Once the materials of the book have been transubstantiated into more magically appropriate materials via its process of clarification, the book has a quality of 4+3+1*3+3 = 13, is considered extremely valuable, and cannot be taken out of a magical aura. Level+quality is still 23.
We can assert that this obsessive mentum magus has had a colleague with a communications score of 5 gloss her book, for a quality total of 14. She cannot turn the work into an inspeximus, as her mentem has not substantially increased during the time of clairification. Specifically, her mentem would have had to increase by 3 points, or 69 xp in that time.
And that leaves me with a summa on the art of mentem, prepared by a maga in her prime, with a quality of 14 and a level of 10.
And we still haven't met the minimum level for trading sound books, despite having invested seriously non-trivial amounts of time and resources into the book, time and resources that make the book functionally ineligible for trade: the very purpose for the book's creation.