Isn't attack and defense on a scale of dimishing returns? In that case, a 50% decrease in attack will not necessarily result in 50% less kills. Depending on whether the original attack was 6 or 26, and whether the target had armor 6 or armor 26, you would see different results. Also note that there are certain 'fixed' combat bonuses (like those for flank and rear attacks) that cannot be modded in any way, and will mess up the equation.