After posting a prior question about return grille filters, I ended up opening a can of worms on how my duct is set up. Below is my info and then the questions.
Goodman 3 ton, SSZ16 with a AEPF426016

Main level return is 8x14 at the bottom of the wall in the dead center of the house in an open foyer. Grille size is 16x20 (with 4" of depth)but the duct opening is now cut to about 7.5x18. There is also seven more feet of duct running up the wall before it's boxed off.

Second, 14x8 return runs down from the upstairs hall close to the ceiling. About a 22ft run with two 90s. 16x20 grille (same depth) and about 13x18 duct opening.

Both lead to a 10x20 main duct.
I've been seeing the basic recommendation of 144" per ton.
1) If each duct is 112" then how could I even come close to 432"? Assuming no filters until the 21x23 at the handler, I know that this is an unrestricted 224", but it still seems low. I was considering adding more returns, but will it make a difference in terms of air flow?

Also, I have two, 8x14 supply lines. One to each floor.