Probably the reason you've read many different things is because humidity is not as precise as temperature. What you are really striving for is for the egg to evaporate enough that the air cell and chick have room to grow, without evaporating too much, and the amount of humidity that takes will vary according to your climatic conditions, altitude etc., so to some extent it is a learning curve to figure out what works for you in YOUR conditions.
I can tell you what works for me in windy Kansas. I incubate at 35-40%, and increase it to 65-70% for lockdown. With humidity running at those levels, I am getting 90-100% hatches from my own eggs (shipped is a whole other ballgame and I haven't gone there in a long while).
