What bandpass Q factor do I want for 12dB/octave roll off?
The roll-off characteristic of a band pass does NOT depend on the Q-value.

At first - the Q-value Q=wo/dw is defined for the lowest-order bandpass (n=2) only (dw: 3-dB bandwidth). It is identical to the quality factor of the corresponding pole Qp=Q) 

Secondly, the roll-off value does depend on the filter order only.
Therefore, a roll-off of 12dB/octave is equivalent to a second-order roll-off - to be achieved with a 4th-order bandpass only. (Note that we have a minuimum of n=2 for the most simple bandpass; the filter order is identical to the order of the denominator).    

Comment: As you are asking for a specific attenuation at a certain frequency, it seems that you are mixing stopband attenuation and roll-off characteristics.