I've been fletching arrow now for over 20 years both for competition and hunting. If you shoot fingers you want to keep the fletch about a inch off the tail section for clearance purposes but as for a standard measurement it is what flies the best for your setup. Start at the first 1/4 mark on your clamp and go from there. Most clamps have 1/16 marks on them. Once you find the best setting write it down and fletch all your arrows the same. I have also found that F.O.C. is effected by fletching placement to a degree. Think about it, if you put your fletch all the way back you are only steering the tail of the shaft not the whole shaft. Look at some old flight arrows. Fletching is forward , not all the way back. Broadheads also seem to fly better forward a bit. This does not mean 3 or 4 inches. 1 1/2 " is a pretty good distance forward from the tail of your shaft. Again, try a few arrows first. Smaller shafts require less fletching forward as the size does dictate a smaller fletch all together. Hope this helps you some, just don't give up when something does not work. If everyone gives up and does'nt try something different now and then we would all be shooting tree limbs still with stone points. Keep shooting anything you can as long as you shoot something.