Can someone tell me how much spacing I need around a 1/2 inch round pad and a surrounding
 ground plane with 500 VDC differential  at 1000 feet altitude and a dusty environment?  This is in a high
 power application (15KW).  I've seen answers ranging from 0.0002 inch per volt for non-coated
 boards in an old MIL-STD-275, to 0.5 inch in a VDE handbook.  I need a good practical 
answer and would greatly appreciate a response.