Hi Steve
SBS's 1/48 scale riveter is a lot finer than the Excel one you mention - measuring a sample row of rivets, it's got 40 teeth/inch. 15 teeth/inch seems very coarse - by way of comparison, the rivets on Trumpeter's 1/48 scale Wyvern and Panther average about 32 per inch.
The spacing of full-sized rivets varies depending on the stress/load applied in any particular area, but often the embossed rivets on kits are clearly nothing like close enough together. I'll try to remember to measure a few sections of rivets next time I'm working on the exhibits at Hendon to get some sort of average values.
All the best
Rowan