@Poettchen if you ever want to give this another try, keep in mind that some patterns, like this one, just use string very unevenly. It would be worth it to count the knots used for each string and take notes.
For example, the B and F strings make a knot on every row so those would have to be much longer than the other strings. And the E strings could be much shorter, in fact one of them isn’t used at all if you follow the knot directions in the pattern. I base my average string length on the length of my arm across to my opposite shoulder to allow for ties on each end. (so if folding in half, the length would be several inches more than my wingspan) Then I cut corresponding individual strings longer or shorter compared to how often they would be used. Be sure to get them placed in the right order before knotting!
But if you do run out, you can add more string the same way you add colors in an alpha. There's a video tutorial on #43509 that might help 😊
xa_vdl
