There is a an interesting article in the summer 2014 edition of the Engine Masters magazine (quarterly spin off of Popular Hot Rodding). I picked mine up last week at the grocery store.
They did dyno testing on a 383 Mopar running different lifters with different preloads, on the same cam. Obviously the 383 Mopar isn't exactly like an FE, but it does use a shaft rocker system. There are too many variables to list that make the testing that was done specific to that exact engine combination, but still a thought provoking read.
Personally for a hydraulic cam, I have always gone 0 lash plus a 1/4 to 1/2 turn on the adjuster.
Brett