One of possible decisions (if your unit is Skydat). Vertical distance between sender and magnet 1-2 mm - check that RRPM display works stable starting from 40-50 rpm. Don't hesitate to pay more attention to small screws and bolts both on sender, wires and magnet: they should be locked additionally or you lose sender and magnet very soon. I use to lock such small hardware in place with silicone sealant such as blue ABRO. Works fine.
Cheers,