I assume you know for sure that the HoE Muted Munro has a sett size of 8 1/4"?

If you want 21" in the pleats at the hips, the waist in the pleats is likely to be non-standard, too (i.e., you won't do the splits like you would for a trad kilt). So, for the sake of estimating, let's say that the waist will be split evenly and have 13" in the pleats.

Pleat size at waist: 13/16"
Pleat size at hips: 1 5/16"

So, I scaled the Muted Munro thumbnail from the HoE site to a sett size of 8 1/4" and put the pleat size on it:



If you want to see what the pleats will look like, you really need to do a composite with tapered pleats. I chopped out the relevant pleat piece outlined by the lines above and composited what the pleats would like with taper. Remember when you look at the image below that, above the waist, the pleats will be straight and not tapered, and ditto below the bottom of the fell. What you see below is as close to being in scale as possible, provided that the sett size is actually 8 1/4".



Dunno if that helps or not.