Friction Rate Chart???!

This may be hopelessly arcane, but I must try! ACCA Manual D for duct sizing
uses a Friction Rate Chart. The x axis is logarythmic. It is for "Friction Rate".
The Y axis is for air volume in CFM.
Lines on the chart indicate Duct size and velocity where X & Y intersect.
I need to find a formula which converts Friction Rate & CFM into Duct size. This is somewhat different from the Darcy Equation because Friction Rate = (pressure drop *100)/ Total Effective Length which involves the length of the duct added to the effective lengths of the fittings which terminate the duct.
Is there anyone out there with experience or occult knowlege with this ?

Any wisdom out there??