Grains per pound as desired by manufacturers is for warranty purposes and is a general guideline. They can't control who buys their bows, what draw length and weight is being shot or other matters. That's why they recommend a set figure. Many are dropping the 6gr/lb minimum to 5gr/lb. That's because they make good stuff that stays together.

Now there is another chart that shows SAFE minimums based on poundage and draw length. Draw length is a big factor. Longer draw means more stored energy in the bow. Meaning that a person with a shorter draw can get away with a lighter arrow for the same poundage due to the fact that he isn't stressing the bow as much to begin with. Look up the AMO (ATA) Minimum Arrow Weight Chart to see this.

Another thing most people don't take into consideration is the weight of all the crap they have on the string. This stuff weighs X amount of grains and is just like having it on the arrow. Only difference is it's on the string for every shot. Of course it's not in the middle of the string so how do you calculate all this? You don't. You get an arrow that shoots out of your bow and quit worrying about the bow blowing up.