The better way is to run a small signal sine wave and check the minimum real motion of the woofer cone at Fb once you have the approximate port length


A number of factors can influence the Fb in reality

(The density and amount of Fill, QL losses - leaks, port air turbulence and location to boundaries
I would then confirm the bass subjectivity in your room

Subtle adjustment of the port output can modify the response to compensate for room placement

The online simulator uses assumption