The 31.5" comes from a derivation that converts the ATA (AMO) 26", 2#, method to the 28", 1.94#, ASTM method that is used for aluminum and carbon arrows.
When I was developing the Spine-O-Meter I used beam deflection theory to derive the conversion factor. The result is that the deflection of a carbon or aluminum shaft measured on the wood ATA standard (26", 2#) is 0.825 x the ASTM deflection. For a 2016: .531 x .825 = .438. So 26/.438 = 59.3. The more direct approach is 31.5/deflection since 26/.825 = 31.5. I have a write-up of all of this math for anyone that gives me their email address.