I have a good red oak board. If I cut it I can make 2 1 1/2 and a 2" wide bow. How do you calculate the appropriate target weight if the bow is made longer. If I want to make one of the 1 1/2" wide at 70" and the other at 72" what kind of weight should I shoot for?

While a was looking at the oak boards, there was some poplar. I was wondering on difused ringed wood,  is it as important for a backing if the board is rift sawn?

ttt