You could rework the mechanic to determine what spell level character get at each level, something like:
class
level - spell level (0,1,2,3,4,5,6,7,8,9) - rough total HrP equivalent
1 - 2 - 6
2 - 2,1 - 11.5
3 - 3,1 - 14.5
4 - 3,2 - 20
5 - 4,2 - 23
6 - 4,2,1 - 31
7 - 5,2,1 - 34
8 - 5,3,1 - 39.5...