Prepare an antibody curve that reaches the saturation in ELISA. Determine the maximal absorbance at the saturation. Calculate 50% of this maximal value. Interpolate this 50% value in your curve and detemine which antibody concentration corresponds to this value. This is a way to compare binding activities of different antibodies. The lower this antibody concentration giving you half-maximal value is, the stronger is the interaction. This method is useful, although it cannot replace affinity measurements.

If you are working with polyclonal mixtures with unknown antibody concentrations, then what you plot in the x axis is the dilution factor, not the concentration, but you can also use this approach. In this case, your result will depend on both the amount of antibodies in every sample, and their affinities.