Head to a bow manufacturer site that has 'arrow ballistics.' Find your arrow speed and distance. The Excaliber crossbow site gives the kinetic energy for certain speeds at distances. Mentions that 30 ft lbs is needed for pass thru shot on deer; sounds light to me.
http://excaliburcrossbow.com/content/arrow_ballistics
Another issue related to "effective range" has to do with sound speed. Though I've never hunted 'whitetail' a read of veteran bow hunters generally allows 30 yards; further distances risk your deer jumping the string. Though 50lbs doesn't seem to be in the fast/powerful range for bows, it's likely powerful enough out beyond your ability to place 'a humane' shot, assuming ethics is part of your 'effective' definition.