The analysis is good, but he doesn't necessarily want to damp 20Hz, that would require extensive amounts, and is difficult to contain with ordinary household construction, so it will be somewhat damped or will radiate away anyway. He wants to damp the higher length modes primarily, above 20Hz, specifically the ~41Hz, ~62Hz and above modes.
He does have a large room working in his favor, and seating placement may be able to avoid problems at the 1,0,0 20.5Hz mode. But this is all theoretical. Very difficult to tell how much of the rest of the house the subs will "see". Only way to tell is to place the subs and then experiment with absorption on the rear wall, or smaller subs in the back of the room.