issue setting up speedo Dash2

[Whizz]

Morning All.

I am having a few problems getting a reading from my RTech wheel sensor. I have a red light on the sensor so getting power, will check later with multimeter to see if it is actually pulsing. Can anyone tell me what the optimum space is between sensor and target . I have 0.8mm at the moment. The target is exactly the same size as the sensor dia.