At a given spine and shaft diameter, many woods will vary as much as 100 grains or more. Shafts will vary as much as 3 gpi or more within a single species of wood, and 5-8 gpi or more between different species of wood, at the same diameter and spine.
If you manage to collect a lot of numbers, you'll end up with a range of gpi for a given wood, spine, diameter. However, that range will be quite large (compared to carbon or aluminum), and of questionable value in designing/creating wood arrows IMO.
But good luck in your endeavor.