N mirabilis has very thin leaves, so humidity should be fairly high (much higher than you would have for ventricosa or khasiana). I don't know what RH% would be ideal, but anything fairly high, like 70% and up, would be good for it. I've read that it likes constantly wet soil, so you might want to have soil that doesn't dry out very quickly, or maybe water more often. I'm growing it outside with all my other lowland neps (same water, same temps, same humidity) and it seems to be doing well so far. I'm keeping it in an 8" or 6" (I forget) pot. I'm experimenting with potting mixtures right now, but I’ve used mixtures of LF Sphagnum, peat, vermiculite, charcoal, and lava rock (2:1:1:1:1) with no problems. I’m trying to see if a mix of LF sphagnum, river gravel, peat, charcoal, and vermiculite (2:2:1:1:1) will do well too for all the plants.
Joel