spine starting point, moving from carbon to wood

[Arrow Finder]

Hi all,
IŽve got my head spinning trying to workout the equivalent wood spine for the carbons I'm currently using on my current bow, any pointers would be gratefully received. IŽll try and give all the relevant information but I'm bound to haver forgotten something important.
My intention is to gap shoot this bow, shooting 3 under, my current arrows are point on at 20yds with no left or right deviation, just how i like it, but to shoot in the NFAS primitive class i need to be shooting self nocked wood arrows (fair enough), hence the change.
The bows a bamboo backed hazel long(ish)bow, 60" ntn, pulling 46# @29", no shelf but very narrow at the pass.
The arrows I'm using are from my recurve, 30" 498gn (100gn insert, 125gn point) 340 spine with triple rayzr fletchings, no helical.

I'm trying not to sweat too much over this as this bow came in under weight, not that I'm annoyed, its my first attempt at bamboo backing and the hazel selfbow had a nasty lift  and 2" of string follow and had been retired quite a while ago (the bamboo was liberated from screening in the garden so had to use two of the narrow strips side by side), sheŽs pretty ugly but shoots better than she has any right too.
If i use the easton charts 340 spine is equivalent to around 65#, but that doesnt take into account of having 200 and something grains hanging out the front......
Thanks in advance
Tim

[archeryrob]

I did not read anything about a "Spine tester" and you can find plans here to make one. You need to read on bare shaft testing. Make some arrows that are 45# or heavier and tests them unfletched with just points. I typically shoot a 42# spine on a 55# bow. Static spine is calculated at 28" and you are shooting 29", so you loose 5# in spine for the extra inch. Say if you needed 10# under, you need to shoot for 5# under at 40# as 35# will be too weak in spine with the extra inch added in length. Every inch added drops 5# and every inch short adds 5# in spine. Arrow points medium is 125 grains and every 25 grains of change makes 5# difference. 100 grain point add 5# in spine and a 150 drop 5# in spine. 

I shoot my arrows and use a thumb plane and sand paper in my hand to reduce the shaft and lower spine until they fly properly. Then spine them and copy the spine. Most will be good and a few shoot off, but I never figured out why those few become flight play arrows as I can't hit a tennis ball at 20 yards with those few.

[BowEd]

Well stated Rob.
I would add that using a stiff fast flight string can call for a slightly stiffer spine also.

[Arrow Finder]

Thanks for the replies, I've been thinking about a spine tester for a while as I've got a good supply of bamboo locally that can be scrumped without upsetting anyone, be good to use the stuff too thin for atlatl darts.