Is there a rule of thumb for setting the HPF based on the lower 3db limit of the speakers?
No, it's a mixture of your room, your speakers, your sub, your amp capacity, and your listening material/level. If the sub can play high and it's positioned well, you can run a higher crossover point without much localization. If you have an inadequate amp, even if your speaker can play low, you shouldn't cross low. etc. It's a complex equation that's best served with good judgement, trial and error, and some luck.
Generally, you'd like to put the HPF above the -3dB point. 1 octave above or more, even with the best equalization, if you want a maximally flat output (unless you're following THX guidelines and are using THX-adherent gear) to prevent the out of phase port output from meddling.
In most cases, I set it about 1/4 - 1/2 octave higher than the -3dB point as a compromised starting point, if the speaker and the speaker amp are adequate, and the sub can handle the redirected frequencies. Tapos tweak up/down from there na lang depende sa results. This seems to work fairly well with most reasonable systems in less than ideal (typical) rooms.
