Looking for some basement HVAC advice. Considering adding a few supplies and possibly small return near floor to help condition and circulate air. Before I just start hacking into duct I'd like to have some feeling that I'm not going to completely throw off balance...

Using some basic CFM chart guides for common duct sizes, which I assume calculations are based on some reasonable velocities/pressures to provide some ballpark reference..
5" round - 50 CFM
6" round - 85 CFM
8 x 12 - 400 CFM
10 x 12 - 550 CFM
14x4 - 180 CFM (stud cavity)

Current system. ~1200sqft ranch, typical 3bed 2bath, kitchen and fam

- ~1200-1400 CFM furnace/blower capacity (will verify) centered in basement
- 9x12 trunk ~15ft each direction (4x8 register cut ~10ft down one leg that I plan to seal)
- nine 6" supply ducts (one each bed/bath, 2 kit/fam)
- two 6x30 (4 stud cavities along same wall) returns in upstairs hall and ~16x16 (1 stud cavities) return in bedroom.

from duct size chart
~950 CFM trunk capacity
~765 CFM actual room supplies,
~900CFM return capacity

NOW, I'm not too worried that fan spec and chart calcs do not match up, just means that if fan is actually trying to push 1200CFM, air may be traveling at little higher velocity than assumed/recommended in chart calculations. Good news (I think ???), is it looks like return is approx sized to supply trunk, and actual takeoffs are less the trunk capacity?

finish basement layout,,, basically same as upstairs except kit/fam combined into 1 large living area, 3 10x10 rooms, 1 bath, and 8x14 laundry/maintenance with furnace/HWH.

So now my goal is to add few supplies (maybe 4" to create back pressure for upstair supplies) and small return (1 stud cavity) low in basement hall tapping into upstairs hallway duct between joists, hopefully keeping basement positive (greater supply than return, more on furnace vent backdraft below) while maintaining overall balance with chart ratings in the realm of 950 cfm trunk capacity by maintaining return/supply ratio.

- 4" supplies (~25-30 CFM each) looks like I can add ~185CFM or ~6-7 4" take-offs in basement before reaching trunk capacity, thinking one in each corner, (2 in large living area, 1 each in back 2 rooms), and maybe one in center room and bath..
- single stud cavity return - 180 CFM

Man, 185 supply and 180 CFM, thats close, Guess I could add more supply (5") to basement but don't want to exceed trunk capacity??? OR return vent will have 12x4 damper register which would be more like 120-150CFM that can be closed if necessary. What do you think? FDoes this make sense?

Also, the concern of furnace/HWH vent back draft. With current setup and furnace running, I observed air being pulled downstairs (tissue under door), which doesn't necessarily mean basement is negative, but not balanced with upstairs which seems to me that combustion air (or return leaks) is drawing air down (which also seems odd since upstairs supply is 85% of return, I would assume air would be drawn up). ALSO note that this was observed with the trunk vent (I now assume for combustion make-up air) was closed, I will open and observe again. Next test will also include tissue at furnace/HWH vent. this is where the return damper will also help.

There is mention of vent requirements between furnace room (8x14) and larger common area for combustion air... Is this req'd, or would under door be sufficient, given I have no plan to condition furnace area with HVAC supply line?