Hi thanks for the picture, yes you're exactly right when I magnify the corner in the cam software I get a corner which is over shot on both lines like your first picture. The test shape I cut was done just using the basic line tool and not a polyline so I think this is the first thing to try.
im going to stick with autocad for now cos it's what I'm used to and cos I have it I obviously tried sketch up but I don't find it that good for drawing tbh we only use SketchUp to take a 2D autocad view and render it in 3D for clients approval.

Chris