Hivi F6 optimal box volume

I may be a little divergent here.

I wouldn't use that driver (or most drivers) at any Vb>Vas. With excursion that small on the F6, one would also be well advised to tune the box higher than Fs as well. The number you need to look at for a ported box is Qa (?) and for small, low power drivers like the F6 it helps to have more box support than sims show.

Relation between Vas and Enclosure volume

I do know a lot of folks have built speakers with this driver, though I'm not keen on the peak approx 1k that needs a notch filter for sure, and the gunk in the crossover stopband that makes it very hard to cross over with anything less than LR4 slopes, or into a very crossover that requires a pretty beefy tweeter.

Zaph|Audio

If you already have a tweeter in mind, it should be crossed around 2k or lower, with a resonance trap on the woofer to help with the ragged octave just above crossover. The SB26STCN is a nice cheap tweeter that would do a decent job, the SB29RDCN is a better tool for the job.