Auto bed leveling problem

Hi all,
I have a Prusa i3 and I have just mounted the auto bed leveling with an inductive sensor.

Now I have some problem to set the correct offset to my extruder. I have the Marlin 1.1.0 RC6.
For example, when I do z homing on my printer, the nozzle is stopping about 3 mm above the bed (the z homing was stopping immediatly when the sensor detect the bed) and I can't to make it closer.. I try to change the value of Z_PROBE_OFFSET_FROM_EXTRUDER but never change

Somebody can help me?
You can see my problem in the attached image.

Thanks a lot