Formula is Box Tuning * 0.75 = SSF setting.
I have a theory that no one who gives a forumula for an SSF setting has any idea where the forumula came from. among the others:
20hz
30hz
5hz below tuning
0.75x tuning
1/3rd octave below tuning (geometrically 1/3rd of an octave)
0.67x tuning
arithmetically 1/3rd of an octave below tuning.
and the thing is, many of the formula's will work for 90% of cases, but are not ideal. basically, there might be a lower setting that would also work, and that would give a bit more extension.
the real value is clearly (primarily) based upon the lowest freqeuncy you can play without overexcursion in your setup. (setups that overexurt at 30hz and 60hz will not be saved by an SSF unless the SSF is set above tuning, which is a worse solution the lowering the power).
basically, if you've got a sub with a 250W amp and the sub's xmax is 30mm peak, even at 20hz the woofer won't overexurt -- so why filter at all? if you get a 500 amp, it may overexurt at 25hz. with 1kW overexcursion might start at 32hz. in each case, the SSF can be set different. or, 35hz could be used universally, even though it artificially limits the lower power setups.