Just a thought. How about the fps setting the wt of the arrow so if you have a 148fps drop the 1 and add a zero 480grain arrow should work. Or 160 fps drop the 1 and add a zero and you have 600grains. Just a thought.
Interesting.
Ashby used used a pound/ second formula that takes both factors into consideration; arrow weight and velocity (fps) then developed a momentum factor that was believed to have the best performance. Momentum is spoke of a lot in his reports as the velocity (kinetic energy) will soon leave the arrow but the momentum of the arrow lives on as it travels to the target, thus making the heavier arrow more productive.
Does make sense, but the factor seems a little high based on the responses in this thread. He also referred to the Pope & Young standard (if you will) of the popular 10 grains per pound arrow weight.
I believe using the 10 grain per lb reference might be a better predictor as the information is easier to obtain in terms of draw weight/ arrow weight rather than velocity and may offer a better predictor as a reference without knowing the bows efficiency, which only a handfull of fps separates any of them anyway. Still should get one pretty close, maybe.
Guess the question of how light can my arrow be and still deliver the results may be the real question. That's of course if your looking for that longer range, flat trajectory, that will remove you from spear distance.