I've recently been learning how to tie some decorative knots. This site is fantastic, but suffers from the same frustration point all resources seem to: not exp

I've recently been learning how to tie some decorative knots. This site is fantastic, but suffers from the same frustration point all resources seem to: not explaining how to calculate the length of rope you will need for a given knot so you can cut it before starting. I can't believe so many beginner tutorials seem to forget this very basic first question of "how do I prepare my materials?".