Camera Separation, in 3D graphics anyway, is a matter of proportion. In this case, it would be best to use the distance between the eyes of a standard model (like P4 male, V2/V3, Don, Judy, whomever). In C4D and LW, I set up a camera rig to accomplish this automatically - two cameras grouped into a hierarchy with the same target for "focus" on a particular object or area. This is the downside of real "stereoscopic cameras" (though there may be some that compensate for this) - that each lens does not angle towards the focal point as real eyes do. Real eyes both cross slightly and change their lens shape to focus.