The 4.5 inch set could be box pleated with one set as the visible panel then one set folded under it at each side, so each complete panel takes 13.5 inches of fabric. You narrow it to the waist, but work to the panel, not pleats

That it the trick which took me so long to fathom out.

The hemline of the pleats is three layers. Wide or narrow, the pleats are always three layers, so there is no using more or less fabric with simple box pleats. I supose that narrow panels would use up a small amount more than large ones for the folds, but it would be fairly insignificant.

With a large set it might look best pleated to the stripe - with a 12 inch set you could have the centre of every set as a panel, 4 inches wide with 4 inches folded under it at each side.