Just learned about this simple technique at our LYS, in the why didn’t I think of that category. No more too long or too short tails when casting on.
Tie ends of 2 strands of yarn, either from 2 separate balls, or from the front and back ends of one ball, and make a slip knot. Slip the knot onto your needle, and proceed with casting on, using one strand as the tail and the other as the working yarn. When all stitches have been cast on, clip one end to only a few inches, and use the other to begin knitting. Voila. No more guessing.