Drivers and modifiers

Hi folks,
i set up my own rig and now i want to add some drivers to drive some corrective shape keys. Everything works just fine and there where some nice tutorials which helped a lot. But now I am facin the problem that I get values beginning from -1.395 (79.9) to 0,274 (15.7) degrees. What i need would ve values from 0 to 1 and I really dont know how to set up my modifier so it (or they) will do that.
Somebody here who can help? I am on Blender 2.78b
Thank you
Bildermensch