I again would not worry about spine,I would concern myself more with easily available materials that are easy to work with.I would go with 5/16th ramin shafts,full length uncut feathers(cut them in half they go twice as far),5/16th snap-on nocks,and 5/16th 100gr. glue-on field points(tapered inside)
My 8 year old son Tyler shoots an 18# @ 20" Mikuta longbow. He shoots 5/16th youth arrows cut to 22",tapered,with a 100gr. glue-on field point. These little arrows fly like DARTS!!!!!
BTW, Thank You Very Much!!!!! For helping to teach the "Young Ones"