Wouldnt it be better to limit the points which you can allocate per characteristic ?
This would lead to a real difference between "-", regular and "+" characteristics.
Lets say, you may max. put +10 on any 1 characteristic and Min. +0:
-: end up between 25 and 35
regular: end up between 30 and 40
+: end up between 35 and 45
What do you think ?