There's a bit more to it. It depends on the blower, blower fan curve, and MUA. You want to maintain a minimum airflow rate in the duct on lowest speed and not have too high of airflow on highest. Too high of static pressure will severely impact the performance and safety of your hood.

thanks for the thoughtful comment. let's assume I just want to re-create what I had before. A 600 CFM vertical blower traveling 25 feet with one 90 degree turn to an outside vent on the side of my house. this worked great. Which of my three approaches would work best?

Thanks

Rectangular ducts have effective diameters that are relatively low compared to the sectional area equivalent. Stick to round where possible.

I would follow all the steps I've outlined here many times for determining the hood requirements, then find a hood and blower combination that meets the requirements, and then have a transition made (if not available). If the hood came with an 8-inch duct interface, and an adapter for 6-inch, I would use the 8-inch opening and transition to seven inches.

But first, the hood damper needs to be selected that mates to the hood. In the example, an 8-inch damper will likely have lower pressure loss.

Did you mean an actual 600 CFM from a 900 CFM rated blower, or an actual 400 CFM from a 600 CFM rated blower?

In general, in the full power mode at the flow rate actually achievable, velocity in the duct should be in the 1000 to 2000 ft/min range, so you can calculate velocity for various duct diameters and blower CFM.

Thank you. This and your previous posts have given me what I need to progress.

Take a look at Victory Range Hoods. They have a 7” model.

Victory Range Hoods