I always go by how humid it is in my room. Under 50, I get nosebleeds and over 70 my room feels and smells like a bog. I used to have a humidity thing in my room with a thermometer. Niece ended up breaking it, of course, but I've lived here for long enough to really know what it's like based on the feel. I keep Mama's tank a little bit more humid because there's always a baby shedding, but Thumbelina swims 10 times a day.