I've seen the formula before - using it I come up with 80x.75 / 12 * 7 (hrs, summer time) = 35. BUT - I often exceed 12v - in good sunlight. My batteries are AGM, Optima's can take more, my controller is set for 14.5 and I've seen that on my XBM monitor. Hence my 40 amp hour estimate.
I don't think I'm pushing the line with 7 hours, and my panels tilt to get more sun.
I'll stick with 40, and track it this year. I didn't have my onboard pc set up to track last year.