Per level? It's the same, 0.5%
The base version is (0 + 0.5*skill_level)%, so ranges from 0.5 to 10%
+ is (3+0.5*skill_level)%, for a range of 3.5 to 13%
++ is (6+0.5*skill_level)%, for a range of 6.5 to 16%