Distance between coil and flywheel

Is anyone able to tell me the correct distance between the coil and flywheel for good ignition. If so, is the distance you measure from the outside edge of the flywheel to what part of the coil. All help would be appreciated.

I've found the easiest way is to use a strip of document folder cardboard between the coil pickups and the magnets.This works out to be between 0.4&0.5mm (0.015-0.020"). The magnets will pull the coil down firmly onto the card strip so you can just nip the bolts up. Quick, easy and consistent.
A business card is a good substitute if you dont have any document folder card.
Hope this helps.

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Black PR50 #52.