Personally, I like my pleats between 3 and 3.5 inches visible per box. The first I made was 2 inches from a 6 inch repeat. I ended up with 11 pleats (making 22 sides to steam, which was a PAIN IN THE $$$$). My current personal projects will have 6 (Carolina), 7 (MacNeil), and 8 (X Marks) pleats per kilt. When I do the other half of the Caledonia (F & K, I think-> 6 inch repeat), I WILL either make it 1.5 repeats or doubled for 3 or 4 inch boxes.

With a solid (2-4 projects down the road for me and some black and some tweed for 2 kilts) I'd play with the numbers to get 7 pleats accross the back, using ONLY the outside/visible portion of the pleat. Then, triple that number for total measurement per pleat.