It depends on the tuning frequency of the box, there is no set value. Reason being that the frequency that the subsonic is at is the -3db point, not the point at which the rolloff begins. So, since an octave is a doubling or halving of frequency and as you increase frequency octaves grow in size, you can't use a specific number. It has to be relative to the tuning frequency. But for most boxes, since they're mostly tuned in the 30-35 range, 5 hz below tuning frequency is fine.