How to calculate the high voltage power line trace clearance
To pick one safety standard- 61800-5-1 (ref IEC 60664-1), for 1200VDC pollution degree 2 and insulation material group III, requires you to provide 12.6mm minimum creepage for reinforced insulation. Look up the relevant standard(s) for the precise definitions of the terms used. 

It's your responsibility to determine what standard(s) your product must conform to and to make sure your design meets all of the constraints simultaneously. 

At 1200VDC consider milling (unplated) slots in the PCB.