I just started the 250 box challenge, so I'm super interested in others answers as well :) For me, I realized that I'm not ghosting very straight, so my VP's have been a bit off. Other than that, I've been trying to get an overall idea of how much off my lines are from converging at the intended VP, or if there is any divergence, and then seeing if I can shrink my general gap by being more accurate with where I place my corners.