I am building a barn for my Kubota L2800, and I will be renting a 3pt PHD to dig the post holes. I will be digging in sand/clay soil, not close to trees, and I want to put my 4"X6" posts 3' in the ground. Do I need a 9" or 12" auger? Also, I'd like to hear any tips, hints, or info on how to...