Previous papers by the author [1-3] pointed to a discovery by Fisher [4] that an equation by Fano [5], when used to predict blast impulse from cased munitions, did not fit the available data. These previous papers showed that an alternative equation for casing-modified blast impulse could be derived directly from an equation by Gurney [6] for the kinetic energy balance between the mass of casing metal and the mass of explosive gases. However, this equation was derived for very ductile casings that are accelerated to their ideal Gurney velocity before they fracture. A previous paper [3] showed how the finite fracture strain of real casing metal can be taken into account in determining the relative blast impulse from a cased charge. This paper shows how, based on previous work by Taylor [7], the equation in [2] can instead be modified to allow for the yield stress of the casing metal and provides validation for this further equation from available experimental data.