I use a shareware program called "Boxplot" - it's quite old, and there are many more modern alternatives. Get one and learn to use it, then you can play around and learn a lot!
Speakers of lower efficiency can go deeper in the same box. That's sometimes called "Hoffman's Iron Law".
Q is a more complex issue that it seems. First off, QT is the "total" Q, usually dominated by the electrical QE, with the mechanical damping QM a much smaller player. But QE is based on an infinite damping factor. When using SET amps with no feedback, QE is increased by 30% to 50% - and so is QT. This makes a big difference in box design.
The mathematically simplest ported boxes are for a speaker with QT = 0.38. But you can tune the box and get good enough results if the Q is in the range of 0.25 to 0.5 - the higher QT will require a larger box and will go deeper. A large QT indicates a stiff suspension, which will usually be less linear. Such a system often has a more constricted-sounding bass. The bass can extend to around 0.4 times the speaker resonance, divided by QT.
For sealed boxes, a QT of 0.5 or less is desired. You can only extend the bass to 0.7 times the speaker resonance, divided by QT; many people prefer to keep it a bit higher. The box will be smaller than the ported box.
Be aware that cone volume displacement (diaphragm area times peak displacement xmax) must increase rapidly as you go deeper. As I said, getting the FE166 down to 50Hz (for example) exposed this weakness; cutting it off at 125Hz eliminated that problem. Small drivers usually have small displacement limits as well.
