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