Agree with Toolpusher and others. Get a flex calculator and then realize that it is only accurate with the flex pulled tight and straight. IMHO in most applications 85 CFM for a 6" Flex is on the high side of reality.
I suggest looking for rules of thumb be eliminated from your list of things to do.
Office and warehouse in both Crystal River & New Port Richey ,FL
The ductolator for flex or metal ,shows what the cfms will be at a "selected" Friction Rate(FR is static per 100 equivalent feet of duct),so without knowing the FR of the duct system,it's really useless.
Order Manual D for ACCA,along with J and S,and never have a duct or sizing problem with your installs again,plus fix all the bad ones you will find.