Bruno, - I wish I had that book

Had it in my hands in German (Sand & Sonne etc) while visiting a LW person in Germany.
(That person, scooled by Rall and others promptly declared that Rall had been the finest shot of the LW because of his deflection skills, while Marseille was simply better up close.)
Anyway...
This has been a teasing debate over the times, and some have doubted Marseilles score seriously - i.e. Duncan Smith.
Shores work is usually very good as I know it, and he has published incredible amounts of data, - with some help.
If one is to dig very deep into the day of Marseille, one will need some maps, distances and calculations. D.S. points is that many of the aircraft lost by the RAF (and SAAF) couldn't even have been close to Marseille.
Would be a lovely time-consuming project wouldn't it

OK, - bottom line, Marseille shot up incredible amounts of aircraft in very short timeframes. Be it 17 kills, or 12 doesn't really matter. It's incredible.
And the spoiler:
AFAIK, Marseille was the "shot" of the group, and the tactic was to cover him completely while he was in the business. That may explain why his claim is higher than the rest put together.