Hi ~ to calculate for a box pleat - measure to get your desired finished width. Divide by how many flat sections you want, for example a valance that is 36" finished width could have 3 equal sections of 12" each or 2 at 18" each. 72" could have 6 sections at 12" each, 5 at 14.4" each, 4 at 18" each, etc. With valances, it is usually more aesthetically pleasing to have odd numbers of sections, but not mandatory.
Add 12" of fabric for each pleat, for an even 3" underneath on each side, add how many inches your return is to the wall twice, for each end, and add 3" twice, for double 1 1/2" side hems.
If your valance will be mounted on a board and going around a corner, or corners, as with a bay window, you will need to make deductions in the finished width.
The easiest way to do this is to lay the boards down on a table flat and position them at the angle that they will be when mounted on the wall. You can then see how much space you will lose from the projection of the board away from the wall. Mark where the center is between the boards (where the angle will be on the wall) and measure at the back of the board what the space is from where the board ends to where the angle is. That will give you the amount that needs to be deducted from each finished width.
For example, if you are using 1 x 4" boards and they will be at a 45 degree angle, you will lose 1 1/2" on each side. So you would deduct 1 1/2" from the finished width of the valance to the right of the angle and also from the part of the valance to the left, before you do your calculation to find out what size each flat section of the valance will be.
You should have a pleat centered right on the angle. These calculations will work for any situation where a box pleat design is desired.
Hope this helps! ~ Suzanne
http://sjonescustomdrapes.com 
Box Pleat Question
