Pro75 6xl case needs at least 44.5 inches from aft edge of rear external thrust ring to forwardmost point on the eye bolt in my 75mm forward closure. Thus, putting this bulkhead's aft edge 45 inches from the base of the rocket seems right. 1.5 inches in tailcone, 24 inches in fin can, so 19.5 inches from base of the airframe section.