What do our engines flow out the exhaust?

[Allnew2Me]

I am researching a new 4" exhaust system for my truck and am seeing that all of the mufflers have a stated CFM flow, or rather 3 separate flow numbers. What does the 5.9 flow at power? I understand that it depends on how much fuel you are burning and the individual mods, but what is a ballpark figure?
I have seen one muffler flow 1000CFM and another about the same size flow almost 1600 dues to internal baffles I suppose.
How much is needed and is too much flow (not enough back pressure) a bad thing? Thanks

I found this on the Cummins muffler site but I must be doing something wrong I can't get it to work, anyone want to take a shot?

1. To calculate exhaust flow (exhaust CFM) use:
Exhaust CFM = Disp. x RPM x E x (Temp. + 460) divided by
C x 941760
2. To calculate intake flow (intake CFM) use:
Intake CFM = Disp. x RPM x E
C x 1728
3. To calculate exhaust flow (exhaust CFM) if intake flow (intake CFM) is known, use:
Exhaust CFM = 2.7 x intake CFM (diesel) or 3.1 x intake CFM (gas)
Disp. = Engine displacement in cubic inches (to convert liters to cubic inches, multiply by 61.002)
RPM = Engine speed at rated HP
E = Efficiency (.85 for naturally aspirated, 1.4 for turbocharged, 1.6 for turbocharged/aftercooled and 1.2 for engines
with scavenging blowers)
Temp. = Exhaust temperature in °F (if not known use 900° for diesel or 1200° for gas)
C = 1 for 2 cycle engines, 2 for 4 cycle engines