Merci, Robert - this answers a question that many of us have had.
Going by your diagram - does the sett have to be 25 to 30 centimeters wide, or will a factor of that work?
For example - inside of a tartan with a 30 cm sett, someone is using tartan with a 15 cm sett. If I understand your diagram, this would work, but one full repeat of the sett would be hidden in the double box pleats. Am I looking at this correctly?