All computations aside (and most of them are probably very correct), why
not just use the recommended minimum weight from the manufacturer as an
actual minimum?

Spoke to a pilot the other day who didn't, because his computations showed
less was also possible and ballast weights were not readily available. Got
into an unintentional spin that flattened out rapidly and from which he
recovered at only 300 feet, just enough for a flare and off-field landing.

My point: stay well on the safe side, and don't make a habit out of
undershooting the minimum recommended weight limits... Just because it is
possible to do things, doesn't mean you have to. ;-)