Calculations are pretty easy. That value you determined, 10 + 2*PL, that is the Average CR. It is the average, b/c that is the average amount, so it is still a 50% chance in most cases.
And actually, lower level banes are even easier now, since someone can argue using any attribute, they are going to most likely be using one of their higher stats.
And since there are several banes that never increase in difficulty, those will be way easy to just break out of.
dice = Avg w/ Explosions (rounded down for simplicity, few of these could be rounded up by 1 b/c of +0.5, click on âgraphâ to see the numbers more quickly: http://anydice.com/program/c643 , and yes, it isnât as exact as what OL does, but close enough for these purposes)
d20 = 11
d4 = 3
d6 = 4
d8 = 5
d10 = 6
2d6 = 7
2d8 = 9
2d10 = 11
3d8 = 13
3d10 = 16
Attribute 0 = 11
Attribute 1 = 14
Attribute 2 = 15
Attribute 3 = 16
Attribute 4 = 17
Attribute 5 = 18
Attribute 6 = 20
Attribute 7 = 22
Attribute 8 = 24
Attribute 9 = 27
Part of the reason to group together the PL of the banes, not just for simplicity, is b/c there are banes out there for PL 1, which would be 12 to break out of, so 45% chance IF you use an Attribute Score of 0. At level 1, you are likely to have at least an Attribute Score of 4, which means on average you should be hitting around 15~17 with that attribute.
By making all PL 1-4 being a CR of 18 (by that forumla), you are at least making it a less than 50% chance to break out of it at Attribute 4.