Is it passive or electronic? Generally the frequency setting given for a passive filter is the -3db point, meaning you are -3db @ 20Hz (and -24db @ 10Hz.) Electronics list the settings in various ways depending on the make/model of the unit. What Xover are you using?
Depends on the type of 24dB filter. Most common in this situation would be Butterworth or Linkwitz-Riley, probably in that order. Butterworth is -3dB at corner frequency, and L-R is -6dB. Rest of the frequency response is asymptotic to the ideal you allude to, flat in passband above corner, 24dB/oct slope below corner. Just have to smoothly add in that corner point, and you pretty much have it. If interested, look up the concept of damping and how it relates to frequency and phase response.
EDIT: Also, Butterworth and L-R are just two discrete points in a whole continuum of possible 24dB/oct filters, with damping corresponding to Q=0.707 and Q=0.5, respectively.
After looking at numerous hpf and endlessly modeling a few drivers, but not knowing detailed specifics, 1-4hz below tuning seems prevalent and useful. Between 2-4hz, cone excursion often drops within limits without affecting the breakpoint of the low end too badly. Perhaps someone else will shed more light on this.
To know with certainty you need to model the system and find out where the driver exceeds Xmax, but generally setting the subsonic at approximately the same as the port tuning frequency is sufficient to protect the driver without significantly limiting output.