I would figure out how many rows per inch you get, decide how much longer you want it to be and then equally divide those extra rows between the decreases. I'm aware that this will require math, however.
"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