You lost me here
A rule of thumb - something you'd be well served to go by but isn't set in stone.
set the xover - setting the crossover
at 1/2 to 1 octave - if the frequency is 50hz, then 75hz is 1/2 an octave higher and 100hz is a whole octave higher.
F3 - speakers begin to rolloff at the bottom end of their frequency response. the point where it's 3db down is usually considered the lower limit of useful bass. So if your speakers are 40hz - 17khz +/- 3db then 40hz would be the F3
thus the ideal crossover point would be someone near 60hz or 80hz
According to speaker specs (including CM8's, CM Center & CM1's) the Crossover frequencies are 4kHz. I'll assume this is the same as 40 Hz ? This is what is set in the receiver at this time.
Wait! 4khz is 4000 hz! Not 40! That specified speaker crossover frequency is for where the midrange driver hands off to the tweeter! What you want to determine is where to hand off the subwoofer to the midrange driver! Find the F3 point of your speakers, not the crossover! Here I'll do it for you
Based on the specs your F3 points would be
CM8 - 69Hz
CM Center - 55Hz
CM1 - 55Hz
Why the center and bookshelf extend lower than the 3-way tower is beyond me. Based on that though you should experiment with a 80hz, 100hz, and 120hz crossover points. If you've got stereo subs then higher crossover points are more acceptable, but for most setups 80hz is preferable because it usually keeps the subwoofer from being localizable. You've got your system setup well if
1) At your seating position, there isn't too much or too little bass
2) You can't tell where the bass is coming from if it's directed to the subwoofer. It should really come from the entire room around you in a sense.