Inductive/magnetic sensors for crankshaft, camshaft or speed usually have a gap between 0.5...1.0mm. You can't do anything wrong, start with 0.8mm and check how even the gap around the circumference of the trigger wheel is. What you should aim for is a correct reading on the Speedometer under all environmental conditions. If you experience dropouts or false-reading on the Speedo, reduce the gap to get stable results.
Peter
