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.