There is one method which says "triple the length as you would like it be when its finished". I think it is even from one of the Tutorials from this site.

I tried that once and wasted a couple of meters of Sisal because when i finished the pouch there was nearly no string left. So i wouldnt recommend using that technique.

For Balearic Slings i usually just cut the Strings to a lenght of 4-5 meters each. That way i always had enough to finish my Sling and didnt had to throw away too much material.
Thats for short-to-medium 5 braid Slings.