@julakula i know this is kind of late, but i don't usually measure by cm or in, i use my arms. for narrower bracelets like this one i hold the string in my hand and pull it to the middle of my chest, and for longer bracelets or ones where i fold the strings in half i do my full wingspan (one hand to the other) and that's worked out for me so far!
@julakula if you run out of string, you can attach me string. Alex's Innovations has a video on how top replace broken string that you can use for that.
I have written a tutorial on how to calculate string length btw. You can find it on the tutorial section. There is also one by another user.
I usually cut my strings about 90cm long @julakula
(I also saw your comment on the other pattern btw, but that one is more difficult to answer, because that one is a shaped pattern and those are more difficult to predict.)