I will probably start increasing the humidity today, day 19. Since weighing my eggs throughout the incubation showed an average 11.5% at Day 18, which is in the 11-13% target range, I knew they are in good shape. I will increase it to 50% over the next day or so. Once I have an external pip, I will be more concerned about getting the humidity up to 50% to avoid shrink wrapped chicks.
My Styrofoam incubators have proven to hold humidity well as long as I don't open them. But in past hatches I have learned that I prefer to inceease the humidity slowly to prevent over shooting the target of 50-55%. It's easier to increase slowly than to remove water after too much has been added.
I live in Florida, and right now the relative humidity has increased to 90%. The humidity in the room has already increased 10% (to 40%) overnight, and the incubator % has increased by 2% without adding water.
Take notes, measure how much water you add and how it affects the humidity in your incubator. It will help you with future hatches.
Good luck!