0 stat points to 11 = costs 1 of those 3 points you get per lvl up
11 stat points to 21 = costs 2 of those 3 points you get per lvl up
21 stat points to 31 = costs 3 of those 3 points you get per lvl up
31 stat points to 41 = costs 4 of those 3 points you get per lvl up
AND SO ON
so you being lvl 8 in that screenshot makes you have 24 stat points,
-5 you gave already in the other fields, makes 19 stat points left
-the first 11 points given by first category leaves 8 stat points left
-8 points from 11 to your current 15 leaves 0 skill points out of the calculation

Hope its clear on how it works and how much it costs stat wise with the example i posted above
