My usable range is even lower than yours. I can get terpy hits at 4.5, medium brown color at 5 and near-combustion at 5.5. I keep at at 5 most of the time.and here I am not able to go above 6 on dry stem and 6.5-6.6 on water pipe. I never start below 5 either, I don't get much vapor if any at all. So my e-nano range is basically just 5-6 for dry stem, really small range I feel.
ETA: I don't use water, so I have no data for that.