since my sensor was on the side of my fan duct it collides with my Y stepper motor. So I decided to mount it to the back. I used Tech2C's dual bushing holder and added a mount für 12mm inductive and capacitive proximity sensors.
Marlin need to be configured as follows:
#define X_PROBE_OFFSET_FROM_EXTRUDER 0 // X offset: -left +right [of the nozzle]
#define Y_PROBE_OFFSET_FROM_EXTRUDER 63 // Y offset: -front +behind [the nozzle]
#define FRONT_PROBE_BED_POSITION 64
If you are interested in the pictured fan duct, see