Water Bear, please apologize me.
I just try to express a wish as a user of duct work.
Normally I used a duct calculator rule , the same it had been used since 40 years or more back from now.
This rule work on the same friction loos expresed on inch of water column by 100 ft
Usually a fixed valued is used as per economic reazon and the allowable presure to be suited by the fan.
A normal value is set from 0.1 to 0.25 inch water column by 100'
This system allow an easy way to know the total pressure loos of a system.
Hope you understand me , my english is poor , because I'm not in USA and it is my second language.