How many cold air returns for a basement room 19' x 35'? It has 3 heat vents in ceiling. I'm assuming at least one at each end of the room would do, or would three be even better? Also, can I use 6" flexible duct tubes to get to the cold air return from the wall pockets instead of boxing in the ceiling joists? And finally, can I connect a 6" duct to the return vertical stack instead of the horizontal run? Due to space, this is the only easy access for one of the cold air returns.

Thanks,

RL