Per page 265 out of 15th edition ARRL Radio handbook, Figure 26.
Usual breakdown ratings of common plate spacings
air gap in inches/ peak voltage breakdown
GAP Voltage
.030 1,000
.050 2,000
.070 3,000
.100 4,000
.125 4,500
.150 5,200
.170 6,000
.200 7,500
.250 9,000
.350 11,000
.500 15,000
.700 20,000