Note: all of the above applies to knife pleat kilts. A similar principle applies to box pleat kilts, but it's less pronounced. Basically, unless you overlap the inner folds of the box pleats (which you absolutely can do, thereby making the box pleats smaller....and getting more of them...AND using up more material) the sett size totally determines the width of the pleat. You just assemble enough box pleats to go roughly round the back half of the guys rumpus, plus or minus an inch or two, and then make an over-apron sized appropriately to cover up what's left.

or at least, that's how I make 'em. Matt and others may do it differently.