I think that if you work until you've completed 9 buttonholes, you should end up with close to the length you want and still have the buttonholes line up correctly at the top and bottom.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry