math behind reducing main trunk size over lenght

I'm just doing some of this in my head and was wondering about how you calculate the reduction of side in a main trunk duct as it gets longer..

does it have to do with the number of takeoffs over the length, or is it just length based?


It seemed like as you take air away you'd want it to get smaller to keep the velocity the same as the amount of CFM would reduce as you add more take offs... is this how it works?

never looked into this, tried to look it up on google, but haven't found much good info