For most applications, a 10" bypass can pass upto about 1000 cfm.
Calculate bypass based on velocity, not friction loss. The Bypass duct goes from the area of highset positive static to the areas of lowest negative. Also there is very little friction loss for there is very little duct work.
Therefore you get a lot of velecity through the bypass damper.
@approximately 2000 ft/min allows 1100 CFM through a 10" damper.