Try this, Sit bone measurement = Y,
Y + 2(15mm)
where (15mm) is the distance from the sit bone to the perimeter/edge of the saddle at its widest part or a bit over 1/2" from sit bone to saddle edge which should center the sit bones between the cutout(if there is one) in the center and saddle perimeter edge.
I think that will get you pretty close. I believe that is a Specialized formula.