I also live in a fairly dry climate, south central montana is mostly semi arid, humidity usually hangs in the low teens most times. Since I have been hatching so long I don't wory as much about humidity as I do air cell size. Although I can't really run a dry hatch, I do open all the vents halfway in the back for lots of fresh air and when I candle I make a note of the air cell and compare it to where it should be at the stage it is in. Eggs should loose, I think it was 18% of their volume, over incubation? You can weigh the eggs to get an idea of what you are looking at, I just know what they are supposed to look like now so I don't weigh throught any more, I just candle and look for irregularities. In my incubator, there is only one water pan, with the vents all halfway open. In the hatcher, there is one vent in the back and it is all the way open, but I have 2 pans of water in the hatcher. The humidity doesn't get to over30%. Marans eggs are less porous and so retain more moisture, requiring less humidity that the standard egg. Whatever their setting, it seems to be the same for turkey eggs, because both Marans and turkeys both seem to hatch the best in these conditions.
