To quote the sneaker folks, just do it
. Just pretend that one end of the needle is the end-- it can help to put on a point protector to that if you have enough sts, they won't slide off as you're casting on. The rule of thumb is to have about an inch per stitch for your "tail". Better to have too much at the end of casting on, than not enough. You don't want to get to stitch 217 and realize you ran out of "tail".