So wouldn't you want the crossover point on the fronts to be set at 150 Hz as well?

Not necessarily.

Speaker response isn't flat across the board. Different frequencies will play at different volumes depending on the speaker. So you might want to overlap the crossover points to give things more punch at 150-200hz (a common frequency that needs to be boosted in cars) so that both your front speakers and the subwoofer have some coverage in that range.

It's not a precise science in most cases. It's all about tuning to your ear. The idea is to get it sounding, via trial and error, as good as you can with a flat EQ. Fiddle with the crossover points until it sounds pretty good and doesn't distort. Then fine-tune with an EQ.

You can't work out what something is going to sound like from looking at the spec numbers. You can't plug a math formula into and get your crossover points. It's gotta be done by ear. For example, Doug Burnside runs his subwoofers crossed over at something like 3000hz, which most people would scoff at, but it happens to be what sounds best on that particular system.

This would block anything below 150 Hz? Or would that block above 150 Hz? I guess I'm a little confused about how crossovers actually work.

Crossovers are either high-pass or low-pass, depending on which of the two things you want to do.

A low pass crossover set to 150 hz would be for a subwoofer, i.e., it would only allow the lowest frequencies below 150hz through. A high pass crossover set to 150hz would be for the front speakers, blocking the low frequencies and allowing the highs to pass through.