Off Axis Projection issue

I'm trying to implement off axis projection, for the purpose of a cave type system.

I've used this method https://en.wikibooks.org/wiki/Cg_Programming/Unity/Projection_for_Virtual_Reality, however I'm having and issue when changing the z-value of my camera position. In the video below notice how when I move the camera towards the cubes they appear to shrink in size. It looks like there's some scaling error in z.

Has anyone got any experience with off axis projection? and might be able to shed some light on what might be the cause.

Thanks