RE: calculate arrow speed
You can do it if you know the IBO of your bow. Let's say your Parker Buckhunter has an IBO of 286. That means, with a 30" draw length and 70 lb. draw weight, a 350 grain arrow would be going approximately 286 fps. Going down to 55 lb. is going to cost you about 30 fps. And the 392 grain arrow is going to cost you another 14 fps. So your speed is going to be in the neighborhood of 242 fps.