Laminar air flow in a 2D channel - how to ensure that a flow is fully developed?

Addeeellll

Hello everyone,
I am studying laminar airflow in a channel (2D) and trying to simulate it with FLUENT software but I need to know about the entrance length, and I want to know how I can be sure that a flow is fully developed? is it just by looking at the velocity profile? or looking at the temperature contour?
I am tracking an article which has done numerical study for a smooth channel and a baffled channel. the Nu number for a smooth channel (100<Re<600) is around 7.54 and I am trying to reach this value but I should make sure about the Entrance Length first.

the formula for a laminar flow in a channel to obtain the entrance length is: l=0.05*Re_D*D_h
but this formula will not give me the desired results.
would anybody help me?
Thank You