Aspheric surface testing would be greatly facilitated if the requirement for a null condition were removed. Testing an optic in a non-null configuration introduces aberrations into the wavefront. The wavefront measured at the sensor is different from the wavefront initially produced by the test surface, and the interferometer must be calibrated if useful measurements of aspheres are to be made. One potential calibration technique is reverse optimization, where a lens design program is used to retrieve the prescription of the interferometer. Various problems in modeling an interferometer, and potential solutions, are discussed. A defocused sphere was used to generate a non-null wavefront with 100λ of departure at the surface. The reverse optimization results matched the experimental data to better than λ/4 PV.