I’ve made many yew sap bows, but like DC, all have had heartwood on the belly. Some, the sapwood curves around the sides of the limb, and I try to leave a swell of heartwood in the centre.
I always aim for less than 40” on these kinds of bows, I feel like yew bows with more than 50% sapwood maybe take more set, but that’s more of a hunch than real fact maybe.