Delaying driver?

How would I start a bone driven shapekey at a certain point/value.
I have a bone driving a shapekey but I wan’t to start driving the shapekey value only when the bone is rotated halfway and not immediately as soon as the bone start rotating.

After some playing around I managed to figure out a solution by putting a negative number for the second value of the generator and adding the number it back to the third value. It works for my situation but don’t know if its “right”.