I use 45-55% during incubation then bump that to at least 65%, usually 70% as my goal. Even during slow pip to zip stage I have not had any issues with my chicks drying out inside. To get it up that high I add wet sponges and a rag on the floor draped down into the water wells. I add water to the sponges and floor thru the vent hole (which should be left open in lockdown because chicks need fresh air), as needed to maintain the higher humidity.
What is the max? Like, if I hit 80% that'd be ok or too much? Not counting the spike as chicks hatch. I think I got it up to 62%