If you make the long-tail cast on too short, you won't be able to finish casting on. If it's too long, you'll have a long end to take care of. How do you make it the right length? A: The amount of yarn you need depends on the size of the needle and how many stitches you need to cast on. If you wrap your yarn around your needle as many times as the number of stitches you have (plus an extra one per every 5 stitches or so), you should have enough to do a long-tail cast on with a little leeway left over.