Using Operand TTHI should do the job for you. If you take a look of the example, Double Gauss 28 degree field.zmx, in ZEMAX and using the settings shown in the following picture, you will notice the TTHI returned is the distance between the last refractive surface and the image plane is returned. Hope this solves the question you had.
hopebox
2011-01-12 17:56
My apologies, no picture but set surf 1 and Surf 2 = 11 in the aformentioend example.
