High voltage Pin PCB clearance
Clearance is clearance. It doesn't matter what type of conductors you're creating clearance for, whether they be pins/pads, traces, pours, etc. What's important, however, is that you make note of whether the connections are internal, external, or coated. For example, to find the required clearance for between two pins you have to use the number for external conductors, whereas if you have two traces on internal layers you can use the number for internal conductors. In your case you need to find the required clearance between an external conductor and a (presumably) coated conductor (it looks like your board is 2-layer, which means both the top and bottom layers will most likely be coated with soldermask). IPC-2221B states that for 300V DC you'll need a clearance for external conductors of 1.25mm, for internal conductors of 0.2mm, and for coated conductors of 0.4mm. 



Since your ground pour is coated you should be able to use the 0.4mm figure (like you are showing). I generally try to give it a little extra space when I can, though. You seem to have plenty of room, so why not double the clearance, just to be safe?