First-order approximation
A first-order approximation would be:
(MAP/ATM)*DISP*RPM/(2*12^3)
Where:
MAP is manifold pressure at WOT. Best measured at the cylinder intake, not just after the servo to count for induction tubing losses.
ATM is atmospheric pressure (29.92)
DISP is displacement of the engine in cubic inches
RPM is engine speed in revolutions per minute
The divide by two is because the engine only intakes on half of the crank revolutions. The 12^3 converts from the engine displacement in cubic inches to the desired cubic feet. Note that this does NOT convert to SCFM, just to CFM. The S requires standard conditions, which, for SCFM I would argue that there really is no standard.
So, as an example, your 540 cubic inch engine, running at 2800 rpm, at 28 inches manifold pressure, is taking in 409.4 CFM. As a first-order approximation, this should work.