That to me would give vary low cfm estimates. A 408 x 6000 x 0.0009785 / 8 = 300 cfm so a 190cc trick flow head x 0.6872 = 205 cfm a J head. The trick flow seems to be the right call for that application might get away with a little less but not a J head, at 100% ve the formula says 241 cfm so a Edelbrock/SM head.
These equations are for calculating max cfm demand and average cfm demand. Neither one is described as 'THE' formula needed to choose head size. A think they are 'A' formula used to help make the decision.