Driven shape key on multiple bone poses? How can I proceed if I want the shape key to behave differently depending on the bone pose?

I created one and linked it to a clavicle bone to affect it when it's rotated on z-world space(it was the only one that worked)

However, It will mess up deformations when i rotate the bone to the opposite direction affected by the shape key.

I've also tried using 2 driven shape keys linked to the same bone to correct 2 different poses, but it seems one always topple the other.

Any help is appreciated :)

EDIT: I found out what's wrong. But now I need to modify the driver so that the shape apply ONLY between 0° to -20 degrees. I'm almost sure I have to tweak the curves, but I simply can't understand how that graph works.

Here is my driver setup(without touching the curves):



How can I change it to work in the 0° ~ -20° rotation range?
In the "generator" section of the dope sheet panel, you want to adjust the x-coefficient of the Expanded Polynomial. 

You can see a visual explanation starting around time 11:30 in this video on corrective shape keys.