Question: how do you know how much yarn to use when casting on?
Answer: If you need 20 stitches of 4mm diameter stitches, then you'll need a minimum of 20 x 1.26cm length. Convert to inches (1” per 2.54cm) and you've got roughly 10” minimum for the tail to cast on. Add a few more inches so that you'll have yarn to hold on to at the end; four to five inches should do it.