The ear test is best but for rules of thumbs there are two main ones.

1. Set the crossover between 1/2 octave to a full octave above the minimum frequency response of the mains. So a speaker with a 40hz FR would be crossed at 60hz to 80 hz.

2. Set the crossover to 2 times the FR. ( Same as one octave up). 40hz speaker gets crossed at 80hz.

John