Redscale XR on Natura

How to fully utilize Redscale XR? Redscale XR is range from 50-200, but Natura is DX coded camera, it only read Redscale XR as ISO100 film. If would like to try ISO50, then expose the picture with +1, if want to try with ISO25, expose it with +2. How about ISO200? Expose it as -1. If you have a Natura, you will know what I mean here.