FOR THE RECORD

I happened to see an appointment card in the trash so I put the calipers on it. 

The card measured .0115".  

You are going to see this number vary some if you go around measuring all the business and appointment cards in the house.

.010" - .015" and you should be OK? Too close and I'd worry about wobble and too far and the spark may be weak?

This distance does change the timing a minute amount but that is a minor factor.