I ran some numbers for the initial setup I am considering (8'x6' screen, ceiling mounted projector 17' back, fairly central/optimal viewing position about 14' back). With definition 'B' this leads to viewing angle extremes of ~41 degrees; with definition 'A' it is ~23 degrees.
Now looking at the gain curves for a, say fairly flat dispersion/lowgain, Greyhawk; definition 'B' suggests nearly a factor of 2 gain variation over the screen even for the reasonably ideal setup. Intuitivly this seems large (ie. wrong). Can someone else, perhaps even Mr. Stewart, confirm how the gain measurements are made and what the definition is? Of course, perhaps a factor of 2 gain variation is better than it sounds.