PCB line spacing for NEBS 1089 1st Level lightning test.

[Plymale, Doug]

Hello,
 
How can I determine the best line spacing for high voltage pulses on
differential pairs?  
Should the voltage be calculated at protection device's clamping voltage?  
How does the formula change for interlayer vs. top/bottom layer?
Is there any good reference material?
 
Thanks,
Doug