Jake,
You add 5 #s of spine for each inch over 28"-I've read that you also include the portion of the shaft that's in the point. As noted by others, you also need to increase spine for the string. You'll need to increase spine for any point weight over 125 (5 #s for each 25 grains). So, if you want 31" arrows with 150 gr points, you're looking at adding 20-25 #s to the spine as a starting point-that's 70-75 lbs. I also found a website that says you have add 5 #s to the spine for centershot and another 5 #s for each 1/8" past centershot. Then you have to consider adjustments for string type, form, release, etc. You can talk to Surewood about getting lighter weight shafts.