Qtc is defined by you... what kind of end result do you want. Lower Q setups mean larger boxes(high Vb), lower resonant freq. (lower Fb), and also lower power handling.
High Q setups mean lower box volume, higher resonant freq. and higher power handling.
In the car world, a usual compromise between box volume and space available can be found when the Qtc of 0.85 is used. So just use 0.85 as your number, and go from there. Why don't you just use the excel spreadsheets available on this site??