From an airfoil polar to a real wing
Lifting-line theory explains why a wing makes less lift than its airfoil. The tip vortices tilt the air at the wing by the induced angle αi = CL / (π · ARef · e), so the sections work at a smaller angle than the wing is set to, and the force tilts back, which is the induced drag CDi = CL² / (π · ARef · e).
Endplates make the wing behave as if it were longer. The calculator uses the correction measured by Hoerner, ARef = AR · (1 + 1.9 · h / b), where h is the endplate height and b the span.
Instead of the textbook linear shortcut AR / (AR + 2), it solves CL = cl(α − αi) by reading your polar at every angle, so the result stays meaningful near the stall where the lift curve bends.
Where the model stops
It suits straight, unswept wings with an aspect ratio above about 3. A multi-element wing is treated as one wing with the total chord, and the influence of the car body on the wing is not modelled.
Which span efficiency should I use?
0.85 suits a straight rectangular wing: it gives the lift such a wing has in lifting-line theory at aspect ratios from 5 to 10, and it lies in the range found for the induced drag of real wings, 0.8 to 0.9. 1.0 is the ideal elliptic loading that a real wing does not reach. Lower values give less lift and more induced drag.