I hate building bikes...

Yeah, the seat tube is a bit knackered... Not gonna ride with that much post showing, just wondering how much insertion I can get away with, I'll cut off any excess post... I know I should get it fixed properly, just wondering if it'd be OK to ride as it is...

Hard to say really without seeing the thing in the flesh.

Like I say make sure the post goes at least past the bottom of the top tube, preferably an inch or so.