I'm not a kiltmaker, but there's also a very rough technique we use when desperate in costuming. Get the individual in question to pose for two photos, one straight on and one full profile. Shoot them with a normal lens (about 50mm for 35mm cameras and the DSLRs that imitate them; otherwise the zoom setting where the field of view in the photo most closely matches what you see with your eye without looking up, down, or to the side -- neither wide angle nor telephoto). Then calculate the ratio of the measurement you know to the measurement you want in both photos. Average the two ratios, and then run the math with the known measurement.

So, for example, in the front view the waist might look like it's the same as the seat for a ratio of 1:1. In the side view, the seat might be larger for a ratio of 1:1.2. So the average is 1:1.2. If the known waist measurement is 40, the seat measurement will be around 44.

Again, I can't emphasize strongly enough that this is very, very rough. I've never been on a show where we've had to use it and didn't have to do alterations when we actually got the actor -- though, to be fair, we tend to build large and alter on set when we're not certain. I'd use this only as a sanity check since you have a garment to measure.

Of course, the logical followup question is are you expecting the individual in question to wear this new kilt and look good in it? If it's a gag kilt, a rough idea of the seat will probably be close enough.