Most times you will be able to get 2 x 4 1/2 fletches from the one full length feather.
If you want the absolute most from your full length feathers, try cutting 2 x 4 1/2 inch fletches in the 'traditional' shape. You will get more feather profile on your arrows using this shape than any other, as the back edge of one fletch will overlap the forward edge of the next. Actually you will end up with about the same profile area as a 5 inch sheild cut, but at much less cost.
I burn my fletches into shape using a homemade burner.
It must be my Scotish ancestory.
Mick