Your speakers' low-end figure in, a crossover frequency out — with the reason, whether it is the speaker or the 80 Hz localisation limit deciding, and what to check by ear.
The sub setting follows from what the mains can already do.
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A speaker's quoted low-end figure is where it is already 3 dB down, not where it stops. Cross over at exactly that frequency and both the sub and the mains are rolling off at the same time, which leaves a dip. The handover wants to sit above it, where the mains are still flat.
Two rules are in common use — 10 Hz above the −3 dB point and 1.5× it — and they disagree badly at the extremes: 1.5× sends a 79 Hz bookshelf all the way to 120 Hz, well into where a subwoofer becomes audible as a separate source. This tool takes whichever is higher of +10 Hz and 1.3×, which tracks both rules where they agree and avoids the extreme either produces on its own.
Because of your ears, not your speakers. Below roughly 80 Hz humans cannot tell where a bass sound is coming from, so the subwoofer stops announcing its position in the corner. That is the THX and Dolby figure and it is a good default even when the arithmetic suggests otherwise. This tool tells you which of the two reasons is driving its answer, which matters when you decide how far to stray from it.
Room gain at the seat can be 6–10 dB and no calculator can see your room. Play a track with a walking bass line and listen for a note that jumps out or disappears; nudge the crossover 10 Hz at a time. Also try the sub's phase switch in both positions from your seat — one of them will sound fuller through the crossover region, and which one depends on where the sub sits.
Where the sub goes matters more than the crossover setting. The room mode calculator shows which frequencies your room reinforces, and the placement tool covers the rest of the layout.