Freep, the eyeball method is such that even if the 65,000 contracts at April 35 QQQ were positions that were closed, the 37 Pain level remains valid.
All you have to do is scan the O/I straddle chart and walk up the puts until they exceed calls at a particular strike within plus/minus 4 QQQ points of the current range. It's pretty easy once you get the hang of it.
NDX Pain is also 1500 in April, so it supports the 37 QQQ Pain number.