I think I am not understanding something about how the optimization operands OPTH and/or OPDC are calculated.

For example, if I set up a first OPTH to calculate for the image surface and Py = 1, then a second OPTH also for the image surface but with Py = 0, then shouldn’t the difference between the values of those two operands, when divided by the wavelength (with the conversion from microns to millimetres), give the same value as an OPDC operand with Py = 1?