50% to 55% is a good level of humidity during incubation. Temp, 99.7 At lock down, temp stays the same, but humidity goes up to 65% or 70%. Open all vents wide open at lock down. Do not intervene at pipping stage. Let the chick hatch when it is ready. Good luck!
If your humidity lowers with all the vents open increase the surface area of your water. I personally use a cotton sock because it sucks up the water like a wick from a candle. Surface area of evaporating water is what raises your humidity, not the depth.
X2. Yes, opening vents will cause humidity to drop. But the chicks need huge amounts of oxygen to hatch. So as I suggested you can use damp sponges or as Chicken Farmer suggested, cotton socks. Anything to keep moisture high in the incubator at lock down.
I use cotton socks for the ease of cleaning. After the hatch chuck it into the washer and it's clean again for the next hatch. A good hygrometer or wet bulb thermometer is a must to read proper humidity.
One tip for you. I've noticed that when one chick hatches, all the chicks will start to hatch in short order. The first wave of chicks are usually the strongest. I still keep the incubator going for a couple of days after most of the eggs hatch. The "late bloomers" usually are weaker and have a lot of leg problems. Not all, but most of them. Like I said this has been my experience through many egg hatches and a few thousand chicks.