Another way of thinking about box pleats is:

A perfect box pleat, where the edges meet in the front and in the back, is actually three layers of fabric.
So, if you need 24.5" of pleat coverage then that distance needs 3 X 24.5" or 73.5" inches of fabric.
This of course does not take into account the Tartan pattern. The waste almost all comes from aligning the Tartan the way you want.

In Barb's book she lays out the two aprons first. Taking into account getting the stripe she wants centered on the outer apron.
Then she measures to find out how much fabric she has left to make pleats with.
When you gave your figures I just assumed that you would give your figures in much the same way.



In all of this you did not say how you plan to pleat this kilt.
Pleating to the stripe uses on Sett per pleat and pleating to the Sett uses on Sett + one pleat width per pleat.
The nice thing is that you are using a Black Watch derivative so if you pleat to the Strip you may be able to use the element that shows up twice per Sett. This could save you a lot of fabric.

Another way to save fabric is to use black or another dark color on the back of your apron facings, or for the under-apron and of course the waistband could be solid colored.

I would really not skimp on the depth of the deep pleats. These are what allow the aprons to move and hang properly.
As a rule of thumb I use, as a minimum, deep and reverse pleats twice the depth of all the other pleats. Most of my kilt have deep and reverse pleats 10" deep. There is nothing saying that much of that could not be a solid color fabric.