Turn it up as far as it goes (highest frequency) -- 150 or 200 Hz. Since the crossover only filters out high notes and lets the low notes pass, you want it set well above the crossover in your receiver.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8