I usually brace upwards of 6", but just enough to get clearance of feathers as was said. I also use a single loop string and a timber hitch so I can adjust it to get just the right brace. I've had enough of re-doing my second loop to make a significant length adjustment. Maybe if I was using a jig I could hit the mark better.
I do try and keep my feathers as far to the back of the arrow as possible, my fingers aren't huge so I can usually put them about 1/2" forward of the nock and have enough room. I made my last feathers around 4 3/4" long.