We may be at cross purposes, but the RAIM calculation method you are suggesting is at odds with the Bendix King manual I have sitting in front of me.
Ordinarily, RAIM is predicted in respect of the waypoint to which the GPS is navigating, and is predicted effective the ETA at that waypoint. Current altitude is only incidentally a feature of that calculation.
By contrast though, altitude is consulted for instantaeous RAIM integrity (that is, when no destination waypoint is being navigated towards). Failure of the altitude encoder will raise the probability of a RAIM warning because the GPS is denied one of the means of fixing GPS position, however, where 4 or more satellites are received, disabling the altitude encoder will have no effect at all on RAIM warnings.
Try it and see. You'll be surprised. There is much more to RAIM than comparing real altitude with GPS-derived altitude.
2D