I would try a MMP HPH-12 or MMP HPH-24 when using a 300gr XTP in a Knight. The HPH-12 will be better in larger bores. The HPH-24 in tighter bores. You could also use a Harvester Smooth Black (EZ-Load) in place of the HPH-24. I prefer the shorter Harvester EZ-Load which is labeled (for upto 300gr bullets) when im using Harvester sabots.

Older Knights often run from .501-.502 so my best guess is the HPH-12.