Your math seems good, but you will find when you start using the dinky digger, you will not move dirt nearly that fast. They struggle with hard clay. You will have to scrape at it with the raker teeth alot. I'm pretty good on the mini's and it still took me a day and a half to dig a water line 225 feet long X 4 feet deep thru hard clay. It will get the job done, but I would allow an extra couple of days.
