Several years ago I saw a discussion of this on rec.sport.hockey.
 
Someone did an analysis using NHL statistics, and the order of
the powerplay rankings changed only slightly when they did
ppg/minute of power play (including seconds) instead of the
traditional ppg/pp, counting each occurance of a man advantage as
a separate power play.
 
(kinda like slight variations on RPICH weightings... mostly it
stayed the same).
 
        --david