See that steelhead that just broke the surface sixty feet out and 15 feet downstream? How far are you going to have to cast to put the fly right on his nose if the depth of the river is six feet and the current is perfect steelhead holding speed, aka; walking speed? Seventy feet? Eighty? Or the fabled hundred foot cast? Some engineer will come up with a schematics for this I am sure. Good luck with that. I didn't mention what tip is used nor if the fly is wieghted or not, lightly dressed, heavy wire hook or light. Give your best guess given your experience in such a situation.