RE: how light is too light.
My arrows are 28" long and have a total weight of 352 grains. My bow is set to 67# draw weight. That is 5.2 grains/inch. My arrow speed is 298 fps which gives me a KE of 69 ft/lbs. I shoot a cut-on-contact, fixed broadhead and have gotten pass-throughs on all my deer. I also have great BH accuracy out to 40 yards where they group with my field points. This speed allows me to use a single pin out to 40 yards. I can hold it just under the deer's spine and get a double lung hit. I have used heavier arrows in the past and they work fine also. There is just more of an arc. I think if you are hunting any thin skinned game (deer, elk, antelope) that the lighter arrows will work fine. More emphasis should be put on arrow placement and using a quality broadhead.