Humidity is a huge variable. Where are you located? How is the humidity in your area?
Places where its very humid this time of year, like it is where I live in WV, I dont add any water, and it still maintains 35% humidity in the bator. Very dry locations may need to add some water, so they don't get too dry. Higher elevations are even different.
Usually most folks do best somewhere in the 25-40% range. I shoot for 32% as a random number. Then I watch air cells for proper growth. (I candle often).
Egg porosity also affects this number. More porous eggs may need higher humidity to not loose too much too fast, and vice versa...very dense shells need lower. My silkies usually did better with a slightly higher humidity than others, but still depends on their shell.
Then 3 days before hatch day, raise the humidity to around 60-70%. That usually works for me and most folks I know.