The computations are based on the following code:

with the definitions:

beta = sqrt(mach*mach-1.0)

and:

x = pi*arl/4

The wave drag due to lift is:

cdwl = cl*cl*x/4*(sqrt(1+beta^2/x^2)-1)

The wave drag due to volume is:

cdwv = 4*tc^2*(beta^2+2*x^2)/(beta^2+x^2)^1.5

The vortex drag takes the usual form:

cdi = cl^2/(pi*ar)

The effective span efficiency is defined here as the drag of a low-speed ellipse (with the same span and lift) to the total lift-dependent drag of this wing

e_effective = cdi/(cdi+cdwl)