Preferably you'll want to start somewhere in the neighborhood of 70-75Hz and go down from there until you find your preferred setting.
The commonly accepted wisdom is that around 80Hz bass starts to become localized (you can immediately tell it is coming from behind you rather than being enveloped by it) so you'll want to avoid setting it that high.
Assuming that whatever your highs/mids are can keep up with your substage remember to set the HPF for your interior speakers somewhere meeting or overlapping the LPF setting for the sub. Ergo: Don't set your LPF on the sub @ 70Hz but have the HPF for your interiors kick in around 110Hz as that will leave a rather noticeable gap in the overall sound - you'll be missing the lion's share of the range between 70 and 110Hz. How much or how little of that range you miss will depend on the slope of the crossovers in question (how sharply the frequencies beyond the filter frequency setting are attenuated)