Wow.. I can't believe this has gone unanswered for almost two weeks! I'm soooo sorry!
For a mitered corner, I would increase 2 stitches (KPK in one stitch) at the corners every other row. Try to keep in pattern as best you can. When you're improvizing a pattern as you are, you really need to just kinda wing it and do what you think looks best. Good luck!