I am a little confused in terms of some of your descriptive terms. I am guessing that you meant to write 9.12 grains per inch of arrow length but do not want to make any assumptions.
Your best bet is to go to www.bowjackson.com and use his arrow weight calculator. That is probably the easiest route for you as long as you know the make, model and size of arrow you are shooting.
[Deleted]
