Hi my name is Greg
You should set the de-humidistat according to the humidity you want to maintain in the house and according to the outdoor temperature conditions. If the house is too dry at the setting you have now (-20) try setting it a little warmer and see what happens. Also the humidity created in the home will vary depending on things like showers, dish washing, laundry and so on, so all these things will also play a role in what your settings need to be. There are no (set in stone) settings to give you to use. Every home is different. Try different settings and see what the results are, then set accordingly in the future.
Good luck, Greg