i don't think there is a formula as such, but the relative widths of back and belly would depend somewhat on the relative strenghts of these parts. So if you are trapping Black Locust, where the belly is a little weak, you might make the belly 1/4 to 1/2 inch wider. But if you put a willow back on an ipe belly, your ipe would be very narrow, with a much wider willow back.
($50 to the first one who posts a full draw pic of a willow backed ipe)
((I'm home sick today which is why i am responding to every post I can today!))