I want to build myself a spine tester to calculate the spine value of arrow shafts using the ASTM F2031 standard. I do know that I need a 880 gram weight and that I have to measure the amount of flex that the shaft produces when I hang said weight at the center of a 28 inch span off the shaft.

Well, but what then? I can measure the amount of deflection I have then but let’s say the shaft is deflected by 1/12th of an inch or some millimetres. How do I calculate a spine value that ranges between 20 to 80 from that?

Can anybody give me a hint or solution?