Yea, if 40% is where it is naturally in your area it will be much easier to just let that be the RH for your hatch! Try it and see if it works well for the eggs (which is all that really matters) and go with what is easier. I didn't worry with mine until it went under 20% and then only added water to one tray. It worked for me.
Make sure to have several thermometers. I had two, one digital one mercury, on my first two hatches and they always agreed with each other to the degree. I was confident that they were right because they always agreed, even when the temp went up and down. I had a 0% hatch rate on both hatches. I bought a cheapy at Walmart and it showed 5 degrees hotter than the other two. I bought an expensive one and it agreed with the third cheap one! I had cooked the first two sets of eggs. Crap, live and learn.