How far in the hole 318 pistons

Does anyone know how far down a 1972 318 pistons are? In a book I have it states in 1972-1981 they are .056 down from the top of the bore but with those calculations in the Diamond racing compression calculator it gives about 8.5 to one compression but our factory manuals state 8.2 compression ! I tend to think that the pistons are further down in the bore. I would like to find out the real comp before I pull the heads off as I would like to custom order a camshaft but need a more accurate reading on my compression ratio as it may take a few months to get a custom ordered roller cam and I would not like my engine sitting unassembled for many months.