Since your limbs don't change length linearly, and since the curvature differs from one set to the next, I'd say it would take a rocket scientist to figure it out exactly
Or if you had someone to help you, just put the bow in a stringer, brace it with the stringer so your bowstring is at 8.5", then measure the curved distance that is left between your string loop and the limb nocks. That should get you close