If you are getting the thread count from the Scottish Tartans Authority, the count has actually be halved for display purposes, which explains the odd numbers. So that 17 thread pivot should actually be 34 threads. This means that the total thread sequence (according to the STA info) would have 664 threads.

I'm assuming you don't have a ready source for this tartan and would be having it custom woven. If that is the case you can easily specify whatever size sett repeat you'd like and the mill will adjust the thread count accordingly.

On the other hand, if you want to be historically accurate and keep the original thread count (this is a nineteenth century tartan), you will be dealing with a large sett. Not the end of the world, though. Kiltmakers often have to deal with larger than average setts and there are all sorts of tricks and work arounds.

It's those little challenges that are fun. :-)