A snapshot is any shot where you are not tracking and leading the opponent for a shot. E.g target appears static relative to your gunsite.
FWIW, my kills seem to be roughly 50/50 snapshots/tracking shots. I would guess that the mean distance at kill would be around the 200-300 mark, but i compromise my convergence to be a bit further out, so that distance shots are not totally out of the question if i require to hit something at 400 or more. Setting to my mean point, lets say 250, would IMO compromise shots taken at 400 or more to a point to render them fairly ineffective. I think 325 is a good compromise setting which can cater for most scenarios i encounter, but not specialise in either distance or short shots.
Again, YMMV, as i think convergence is dependent on fighting 'styles' etc.