Tom , stick with the 8 Inch Model , leave the 10 & 12 inch models for the Youngins and the Pto off you Ford Agricultural Tractor ! Unless you prefer signing your Autograph with your Left hand lol . Have replaced over 100 residential fence posts and over 1000 Rural Posts in my day , bigger is not always better ! However to answer your question your methology is sound drill the pilot hole and increase incrementally . Hope your in sand and not gravel with hidden boulders lol All the Best .aka Ewalk !

My stihl has a transmission break that snaps into nuetral when I hit something. It is AWESOME! I have a 8" and 6" bit and an extension so I can dig 48". This rig is used for 4x4 and 6x6 post. the 48" depth is required for temporary electric poles. Its hard to imagine bigger unless you are dropping pole beams.

Thanks guys. As you look at the myriad of different ones, you see the elcheapo ones don't have the shock spring. Not for me. A hundred years ago we signed on to be an Earthquake service center. That was in another life. Good to see they are around yet. The Thunder Bay that Lowes has looks to be close and it is supposedly in stock. . I thought that I may have to weld up an extension to deal with an area by a retaining wall. The bigger was going to be for gazebo beams and some weld em up/ paint em up art work I did that has to hold up to the huge winds that I seemed to have moved into.