You were doing well, until you said this Tom. In fact, one of the key limitations of DX is that an f2.8 lens acts like an f4 lens on FX. You can never obtain the same shallow DOF you can with FX. The corollary to this is that with DX you get the same DOF for the same scene with an aperture one stop larger.
This is quite consistent, however, with a crop having less DOF than the file from which it is cropped.