Inlet area nozzle?

[HattoriHanzo]

More question, hope you all dont get tired :)

I wonder about inlet area of the nozzle. How do I calculate or approximate a useful inlet area.
In Rocket Propulsion Elements they only point on the importance(or not!) of "inlet angles". But what about inlet area?

Because the gas velocity in the chamber are not zero, a “plateauâ€