In general I've noticed the following about diskQs.

1.  There are 2 schools of thought to follow for a general rule of thumb.

     a.  No more than 2 per CPU-- that means for every CPU on the box, you can withstand a Q up to 2.  So the Q for a 4-way box would be 8 tops.

     b.  No more than 2 per spindle-- that means for every HD on the box you can withstand a Q of up to 2.  This requires you to know how many spindles are in your raid set, but it's easy info to come by. 

In general, I've noticed that they are both valid under different circumstances.  The first rule is handy for small to mid sized boxes with direct attached storage.  The 2nd is good for SANs and larger boxes, though it will work well with mid size boxes on a SAN as well.

Of course, these are merely rules of thumb to see where you should start looking for trouble.  The tie breaker of course is fn_virtualfilestats.  This is where you'll get your most valuable info.

--Sean.