There is a great "coffee table" Leonardo book that HAS to have that in it,
I'd think. What about the Santa Maria delle Grazie, itself? Do they have a
website with the history of the last supper including the sandbagging during
the war, etc.?
What about books on 1 point perspective? Books that specialize in only
Italian Renaissance?
I'm sure you checked all of these . . . good luck!