Duct sizing for flex ducting

I put this on another thread. But I think this is important enough to start it’s own thread.
Here is a copy of an online ductulator, for sizing duct.
You can plug in the desired cfm, and use .04 as fiction rate for ducts with negative pressure. Use .06 - .08 for ducts with a positive pressure.
TEL is the length of duct in feet. Most everyone will select flex type duct.
In this screenshot I plugged in 100 cfm since a 4 inch fan is rated at 120 cfm. As it shows you would need a 5 inch duct to carry 100 cfm for 15 foot!
This can be some helpful and eye opening information.

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@icky I saw your other post and agreed, my math isn’t what it used to be.
Bad enough, I got the area math wrong, and now the math needs adjusting for optimum (duct length).
Getting it right, is the goal, improving is the journey.
Thank you