I believe the N2 manual recommends a min level of 0.5ppm. I've been targeting 0.5 - 1.0ppm on our tub with N2. Our tub also has a copper ionizer, so I get some additional mineral help. With your ozone, you should be able to follow the low chlorine recipe on page 6 of the Nature 2 manual and get great results. Since you have ozone, you may be able to use even less MPS.
I've been thinking about adding a salt based chlorine generator. I spoke with a couple of people at spachlorinator.com about the TechniChlor and the ColorChlor. I was told that those could be set to generate chlorine in low amounts like I want.