The official clearance between traces is referenced in the below image, however, due to design considerations alternate “Pickering” figures would typically be used that allow our products to be designed.

From historic testing, it is known that external traces that have a 30V / thou clearance are guaranteed to flash over. For this reason, designs should target 10-15V / thou and the clearance calculated as per the following:-

Therefore, with a 300V signal and a target of 15V/thou the clearance would be:-

The clearance formula is common to both internal and external traces. The performance of internal traces will be greater than external traces as the surrounding PCB material acts as an insulator so can in theory have a higher V / thou target.