Primary-secondary spacing

["Paul Mathus" <pmathus-at-learningco-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

     Hey folks,
     
     I'm helping some friends build a big coil right now, and wanted some 
     advice from the big coilers on the list.  My building experience is 
     with my own small 700 watt coil, and this is a 15KVA pig powered 
     monstrosity.
     
     The secondary is 15" in diameter.  What would folks recommend as a 
     good distance between the secondary and the first turn of the primary?
     My instinct is to go at least 2"-2.5" or more to avoid flashover. . . 
     
     
     thanks,
     
     paul mathus