Support your shafts on supports that are 26 inches apart. measure as accurately as you can the distance from the shaft to the table below it. Now hang a weight of about 2# at the center of the shaft between the supports. One of those deep sea halibut sinkers may work. Now measure the distance to the table. The difference is the deflection distance you're looking for. This measurement must be a precise as you can get it!! A .6 inch deflection will be about 43#, .5 about 52# and .4 about 65#.
Howard Hill wrote that he grouped his arrows by shooting. He'd build a bunch of arrows then shoot them and then bundle arrows that grouped together so that his arrows in a bundle would shoot the same.
Also, when you made the arrows, are all the annual rings in the shafts oriented in the same direction?? If the annual rings on some arrows are perpendicular to the bow and some parallel to it, The arrows will show a different spine.