I should know this but some-how it escaped me?when spine is measured on woodies,is that for a full length shaft?or at 28 inches? if measured at full length - 31-32 inches and I want say 50@29bop what spine would I look at? thanks Burt

Woodies are spined for a 28" arrow with a 125gr point. The spine tester's uprights are 26" apart and a 2# weight is hung from the center of the arrow and the deflection is measured. For each inch of arrow over 28" add 5# and subtract 5# for each inch under 28". For each 25grs of point weight over 125gr. subtract 5# and add 5# for the reverse.
For your 31" arrow subtract 20# of spine weight so for 50#@29" start with a 70# to 75# spined arrow...but only if you are using a 125gr point.

Pat's description of how spine is measured is accurate, but I have trouble following the arithmetic after that.

Need to add spine if you add point weight because the heavier points will weaken the arrow's dynamic spine causing it to flex more if you don't. Likewise, if you subract point weight, can subtract spine as well.

The rule of thumb to add 5# of spine for every inch of draw beyond 28 inches generally holds, but there are other things to consider as well.  If you're shooting a fast flite string, would want to add at least another 5# of spine.  Add another 5# of spine if you're shooting a high performance bow.  Another 5# of spine if your bow is cut to or past center.

Without knowing more about your setup, including draw length ( Is your draw 28 inches, plus an inch overhang for 29 inches bop, or do you draw 29 inches and draw the point to the bow's riser.) can't provide an accurate spine recommendation. Your straight limbed longbows, if they were the same draw weight as your Widow, would require lower spined arrows because they're not high performance by comparison and they're not cut to center, and maybe they have dacron rather than fast flite strings. Hope that makes sense.

Thank-you Pat, I thought it was 28" but I was having a Brain F@@@@@ well you know.Burt

I build selfbows and shoot mostly cane and hardwood shoot arrows so I'm not familiar with the other aspects of arrow spinning due to bow performance and configuration or FF strings.
Thanks for clearing that up, Jerry.

PatB,  I have a very old spine tester  and the uprights ajust from side to side.  Are you saying for me to measure the center of the arrow and move the uprights 13" each way to make them 26" apart and then do a spine check?

With the information provided, I get 55# @ 28"=50# @ 29" BOP using a 125gr point. So start with 55# spine and add/subtract for point weight over/under that. Add 5# for FF string, and add additional spine for shelf cut under 3/16" off-center...........Art

Place the arrow on the uprights(26" apart)with the nock end at one upright and place the weight in the center between the uprights.