That is all true but you have a bigger problem due to the 15 degree valve angle. So your rocker shaft height is limited by the length of the rocker arms. That is why you have to ignore the math and just put the rocker arms where they work. Only exception to this is if you're starting with a clean sheet of paper and are designing your own setup. Then you can control the length of the rocker arme and therefore the location of the shaft.