Steve's calculation is a good one and shows why you got 17 pleats in half the tartan. The sett of your tartan is relatively small (just over 6"), and because you're pleating to the stripe, you can get more pleats out of your tartan than you would pleating to the sett.

If you truly have 30 yards, the most sensible thing to do is hack off a piece 8 yards long and the right width and start laying it out from one end. Chances are you'll have some left over, but you won't have to put a join in the middle of the kilt.

Barb