SCS,
I had to go back and review some PM's with Doc Ashby before I shared this...
In emails, I'd asked "what's EFOC and where does it start?"
His reply was that in the testing he and his associates had been doing, they were all in the 25%-28% FOC range... and that 2 1/2" 3 fletch were stabilizing the 190 Griz El Grande ok with even mildly smooth release. He didn't know how low on FOC one could go and still have the effect they'd noted with EFOC...so it had to be on me to experiment.
A guy in New York, Drew Reniff, made me a A&A Feather chopper by altering a Raptor Chopper. He is getting away with 2" feathers, big broadheads and is only shooting 18% FOC arrows.
I chickened out and had him make my chopper for 3", but with some work, it'll go to 2" A&A profile...
I've been messing with the 3 fletch 3"x 7/16" high fletch. I've not tested the 3" with broadheads yet, but they are noticably faster with field points and drift less in a cross wind... I have 22.3% FOC on my GT set up.
FWIW...