Are u using a angle finder and of coarse you know u measure it at the bottom and top and subtract the two and that is your angle right???? Oh of coarse I must be a idiot for asking RIGHT????? U just don't measure one angle or just the top or just the bottom. as per Dave's chassis book,Oh and also 500 hp is no sweat for a 8 3/4 I have 650 to mine-Good luck