I have duct design problem of static regain method. Can any professionals help to solve the problem below?

Static regain method works by keeping the static pressure constant in the ductwork throughout the system.

From my knowledge, static pressure for air duct was friction loss (Pa/m) of duct times duct length.

For the static regain method, the large cross-sectional area of air duct has a large static pressure. I don’t understand how to use area to calculate the static pressure.