It is a very good link, actually all of them are.
(I learned some stuff.)
Problem is, that we have four intake runners coming from the manifold, and that changes the equations.
Also the fact we have a TB butterfly opening & closing that effects the air pulses, as well as intake valve opening & closing.
Not to mention the shape of the runners, valves, intake piping, and the plenum all effect the air pulses.
Its just too hard to nail down one simple equation for intake/exhaust design.
Hell, the technical info in the links is already above most peoples understanding on here (some of it was above mine).
My equation is a "generally acceptable" starting point for design.
Ideal plenum volume for any car is going to depend on the individual engine of that car.
