I am trying to estimate timber volume using the Faustmann model cubic model, Q(t)=at+b(t)^2-d(t)^3 (as described in Conrad, 1999) but my results are inconsistent with the theory. I decided to try the population growth logistic model (that I didn't prefer initially) Q(t)=K/[1+ e^((-[a+bt]) ) ], I got something consistent but then it limits my timber volume to K. I don't know K, so to get results I guessed it to run the model.

Can anyone who has run the Faustmann model to estimate timber volume help me understand my error and how I can adjust my variables to get consistent results?

Thanks!