Looked at two Black Knight birth certificates one was 71.0 lbs 28" 350 gr. 315 fps the other was 61.5 pounds 29" 300 gr 318 fps. So if you adjust the 70 lb up 11 fps to 326 fps as this is the average speed gain going from 28" to 29" on this model you have a difference of 8 fps. Also the 60 lb model was a half pound more over its weight than the 70 lb so I would say WalkswithaGimp is right in there with the 10fps.
BD