Here's my experience with jetting the XT250 for higher elevations. I got the main jet part number from a parts fiche and then I tried variations of that number at www.ronayers.com
to find the other sizes. These jets were used by a bunch of Yamaha thumpers in the 1980s and 1990s.
Yamaha part number - main jet size
288-14343-70-00 is #140
288-14343-68-00 is #135 standard main jet for 2008 XT250
288-14343-65-00 is #130
288-14343-63-00 is #125
288-14355-61-00 is #122
288-14343-60-00 is #120
288-14343-59-00 is #118
I ride from 5000ft up to 12000ft. On long hills and full throttle in 4th and 5th gear, my XT (with #135 main jet) had a bad rich stumble. I drilled some holes in the airbox, but it wasn't enough, so I got the smaller jets. I finally ended up with #122 main jet. I did not make any changes to the needle. The bike runs much better and I'm happy.