Bob
Maybe this will help.
For me, the spines run pretty true if I am shooting a 28" arrow, (that is 28" from nock to back of point, (bop)) with a 125 grain feild point. That meens that I can make a arrow spined 55-60 pounds fly well out of bow with a draw weight of between 55 and 60 pounds.
If I add lenth to the arrow or a heavier fieldtip I will have to increase the spine to make them fly well. I would probably increase the spine by 5# or so for each inch I add and that much if I go to a little heavier field tip. If I do both I would have to increase the spine even more and if I go to a significantly heavier field tip or breadhead, I'd probably bump it up a bit more.
Right now I'm shooting a bow that is about 55# at my 27 1/2 inch draw length my arrows are 20" from nock to bop and I am using 160 grain grizzlys. My arrows are spined at 70-75 pounds. 65-70 would probably work ok for me too.
The point is that you not only have to look at bow weight, you have to look at arrow length and field tip weight also.
Hope that helps.
Brad