Measure your Sett size and divide by 3. This will give you the Pleat width if pleating to the Stripe. To pleat to the Sett you will have to decide if you want your In Pleats to overlap or not touch at all.
This is because when Pleating the to Sett you find your repeat and add or subtract one pleat reveal.

This is why so few kilts are Box-Pleated to the Sett. The large Pleat reveal causes you to use up fabric at a prodigious rate and have the In Pleats overlapping similar to Knife Pleats. Or you end up with the In Pleats so shallow as to decrease the swish to nothing and cause the Pleats to gape open in an unsightly manner.