Thank you gents for your prompt and kind replies.
From the document you linked it looks they were only three: "
Marseille was credited with 17 kills in three separate sorties over El Taqua, Alam Halfa and Deir el Raghat. "
According to Wikipedia (quoting Shores, Christopher F.; Massimello, Giovanni; Guest, Russell (2012b).
A History of the Mediterranean Air War, 1940–1945 Volume 2: North African Desert, February 1942 – March 1943. London, UK: Grub Street.
ISBN 978-1-909166-12-7): "
The post-war analysis shows that the actual results of the day were probably eight to nine destroyed by Marseille with three or four more damaged."
Therefore, an average of three per sortie, plus one damaged; an average of 50 cannon and 500 MG shells per target.
Thanks again.
Cheers,
GB