Aand i detected the error by myself.
For anyone else needing this particular interaction to work for themselves. In the formula box for passive points, the value for the stat is called "this.x"; Make sure to replace x with the short name of your custom stat. And also make sure to spell the short name of your custom stat in the notetag. Here i somehow assumed that, because the index number of normal stats end on 7, then the third custom stat would have an index number of 10. By replacing <GrowC:10:1 > with <GrowC:xtrasat:1 > (where xtrasat is the short name of my custom stat) i managed to make it work.