How to adjust correctly a corrective shapekey?

Hi, I have a corrective shape key for the elbow, and an expression that uses the rotational difference to a helper bone setup as a reference, like the tutorial of rig tips 2 from auto rig pro:

The problem is that when my arm is in rest pose, the shape key isn’t at 0 as is supposed to be, and when I modify the curve the shape key won’t go up to one, doesn’t matter how much close to the bone is, the expression used is “(angleofthearmbone-var)/var” if anybody can help me with it I’ll be very grateful. Thanks in advance.

Don’t use (angle-var)/var – check the video again, he doesn’t use that either. Instead, use (angle-var)/angle.
improvised a blend … with add modifier + bone scale + driver expression.
updata expression abs((zar*(-1))+1).