The Stanford calculator assumes a lift curve slope of 2*PI per radian, and a zero lift angle of attack of zero degrees. This is somewhat akin to a high aspect ratio wing with a symmetrical section. The method is a reasonable tool for investigating the effects of major geometric variables (aspect ratio, sweep, taper and twist) on efficiency. Be careful when applying these results and note that the method is better for higher aspect ratios and lower sweep angles, and WILL NOT model stall. If you wanted to draw a conclusion for your own design from those results, in the linear range of the lift curve (angles of attack lower than stall), you could shift the entire curve by the difference between your airfoil's zero lift angle of attack, this of course does not make up for differences in lift curve slope.

Use it with caution, but have fun with it.