Hey Ben, just saw this post.
I can show you my spine tester next time you come over. I haven't shown it to you yet because it isn't exactly pretty and I haven't spent enough time fine tuning it to be very happy with it.
Just as important to an arrow spine is the arrow weight. That's a LOT easier to figure out. I have a little digital scale that can weigh in grains, perfect for matching arrow weight to bow weight.
As for the formula for arrow spine, I think you're correct about the 26/deflection. You also have to account for arrow length and point weight. Justin Snyder sums it up better than I could from this post about spine:
http://www.primitivearcher.com/smf/index.php/topic,1910.0.htmlFor recurves and high performance longbows add 5-10# bow weight. For selfbows, subtract 5-10# bow weight. For high performance recurves add 10-15# bow weight. For 160 grain points add 5# bow weight. For 190 grain points ad 10# bow weight. You heard right about reducing or increasing spine. Length affects spine by leverage. For every inch of arrow length (string grove to back of point) over 28" you need 5# more spine, for every inch under 28" you need 5# less spine.
Come on over and let's get our shaft cutting jigs made. I'll also show you my shaft tapering jig. Like most my jigs, it's an ugly cuss but gets the job done!