SBM head velocities and there application?

So according to these guidelines a velocity of 260 fps according to it's calculated average CSA seems to be the ideal target. And the formula to figure that is Cfm x 2.4 / 260 = average csa.

Now to figure a port volumes ave. csa = port cc / (port length x 16. 387), Now I've been going off Earlie A's 4.95" as center line port length, is that close to the average of most sbm heads or it's just the speed masters length?

But if it's close average length the formula is just ave. csa = port cc / 81.12