I have found that lots of archers try to use too littel fletch on their arrows , shooting broadheads . I like 3 5inch feathers , or 4 4inch feathers for fixed broadheads . Trying to tune a big cutting fixed head with , say 3 3inch vanes is fustrating , and futile .

Straightarrow has given good advise , I see lots of archers shooting 29" 400 spine arrows at 70 pounds , way underspined