Choosing an optimum XO point in a FAST requires weighing off a number of factors. Choosing to do a passive XO puts some constraints on the woofer & the FR, and once getting below 200-250 hz becomes next to impractical as the parts get so large that you can just get another amp & biamp for similar or less money.
One of the typical starting points is to put the XO in or around the baffle step F3 (i usually generalize to somewhere between the BS F3 or 0.707 x that frequency). If you are trying to do passive then the woofer should be 2-6 dB more sensitive than the FR (how much is needed depends on the speaker design, the room, and where in the room you put the speakers).
That said i will startout with the XO as low as i think i can get away with (i am usually bi-amping).
With the above we started out at 160 Hz (with a PLLXO), which sounded fine, but was a bit thin in the mid bass, so we moved it up to 240 Hz, where they were judged outstanding. That is still quite a bit below the BS F3 but it seems to work.
dave