That's the approach I always take, too. Because I just HAVE to know. Speaking of, I ran out of yarn this week for my Seraphim shawl. I hope you have better luck.
Makes sense to me, in a nonsensical sort of way. :) Good luck!
Jackie Fee, author of The_Sweater_Workshop, says that if you think you're going to run out of yarn....KNIT FASTER! Should work.
I ran out with my birch, but the lovely Woolen Rabbit Kim bailed me out by sharing her fourth ball in the same colorway with me. My Birch, however, wound up so big I had to give it to 6'4" Juno because, well, it wanted me to.
I did Birch last year, used 3 1/2 skeins but I did it on six 8 needles and I knit very loose, it was huge, if yours doesn't look huge you should make it on 3 skeins. good luck
It sounds like a reasonable course of action.