dig this, i had a settup that was shooting a little stiff, a 75#skookum longbow w/some fmj 300's,the arrow weight was perfect, brace hight ideal, just a bit stiff on the spine,got finelined on my form,with my elbow in line, ok, but borderline. so i refletched,move my feathers about 5/8th" forward from my usual location,went four fletch this time, and guess what, spine got softer even with the added feather,i'd say it about as close as need be as i'm a hunter not a bean counter,lol, but anyway,i'd always considered more fletch to increase spine, never realized how moving the feathers forward drastically affect spine,almost as much as trimming an arrow up or down .hope that goes to practicall application for you all. seasons near....bucks must be nervous