The real issue is not height of sonotube to the depth of the footing but the depth of the footing in relation to the width of the sonotube.
Force from a pier is directed downward and at the bottom of the pier the force is then dissipated straight downwards through the footing as well as at a 45 degree angle downwards and outwards from the outside walls of the sontube.
So, the rule of thumb is: Make sure the footing does not extend outwards from the pier or sonotube more than the depth of the footing.
Otherwise, the angled downward force can break off the part of the footing that extends beyond this 45 degree force line.
So if the footing is 8" deep, it should not extend more than 8" beyond the outside wall of the pier.
Tough to explain in words but simple with diagrams.
I am building a 10'x12'deck (not attached to a ledger board). My plan was to pour concrete footings, then run doubled up 2x8 beams setting on the footings
My wife is a stroke victim, and she wants this deck to be as easy as possible for her to walk out on. She would like it to be as high as possible, with