I'd pleat to the sett on this one (1.5 repeats per pleat), simply due to the size of the sett. If you pleat to the stripe (1 repeat per pleat) your box pleats will end up being way too shallow and you'll have a devil of a time with them hanging right. If you pleat to the stripe (2 repeats per pleat) you'll end up with super-wide box pleats that just won't look as aesthetically pleasing. My solution given a sett of this size is typically to go with 1.5 repeats per pleat, which in a traditional box pleat, ends up looking like pleating to the sett.

M