If you have some shafts that already work, you can make a tester almost any way you want and just compare the bend.
Realize though, that most of the testers I have seen use a system that magnifies the amount of deflection so you can actually see it.
By that I mean, it may deflect .200, .250, .300, .350, .400 and you just might not see that much difference in the direct deflection ( there is not that much difference between those numbers). That is why you have that wire pointing to a set of numbers and why it is offset so that 1/4" on movement at one end shows as, heck 2"-3" of movement at the other.
Back to the point. The numbers don't have to mean anything except as a comparison. If you have an arrow that works, compare the deflections against that, whatever it is.