many years ago I copied a diagram for a low tech spine tester designed by Eldon Bingham. It was essentially the same as the 2 nails in the fence except it used a 30" span and a 3 pound weight.
Here are the listed deflections for various weights. the measurments are from one 5# step to the next starting at level.
-9/16=75#, -1/16=70#, -3/32=65#, -3/32=60#, -3/32=55#, -3/32= 50#, -1/8=45#, -5/32=40#, -5/32=35# I hope this helps some of you who dont have an arrow to compare with
God Bless,
Buck