You can certainly design your speaker boxes, but I wouldn’t.
I’m an installer, not a speaker designer. I’ve installed many speakers, but I wouldn’t like to try to design them. The internals of a speaker box (cabinet) is complex.
The simple calculation of speakers in parallel is only useful if your don’t use a cross-over circuit (a circuit which splits the signal into different frequencies to best match each speaker in the box).
The complexity of the total impedance of a speaker (box) is not the only issue. Other things I’m aware of (among other issues) that need consideration are:
- The internal volume of the cabinet (less the volume of the other speakers) needs to match the characteristics of the biggest speaker
- The effect of the air movement caused by one speaker might have on the other speakers
- The matching of the crossover circuit to each of the speakers
- The phasing of each speaker relative to the other speakers
- The overall flatness of the resultant frequency response.
- The power handling of each speaker
So, I leave all that to the people who get paid the big bucks to design speakers. We need to simply install and enjoy the product of their effort.
