Talking to myself again

Do you do that, too?

I've got 5 balls of yarn for a square shawl that needs 7. So, I'm not making a triangular shawl, and am trying to use as much of the yarn as I can, because it's really pretty.

I now think that I can use 3 balls on the body and knit 3 border repeats, possibly 5, and then crochet cast off everything. So essentially, I'm OK until I have to start the pick-up for the border. I just keep redoing my math, trying to make sure that I don't do something that I'm not supposed to!