The gap should be the thickness of a standard cardboard visiting card. The reason most people use a visiting card is that it avoids having the measuring device attach itself to the flywheel magnet, which makes the job more difficult if it happens. The gap should be measured between the magnet and the coil's laminated iron yoke, not between the outside of the flywheel and the yoke. The gap is not critical, anything between say 0.012" and 0.018" will be fine, but do make it the same for both arms of the yoke.
