I incubate at 45% (the figure I have found will reliably get my eggs to a 13% weight loss as required) and I go up to 75% for lockdown. If the humidity goes higher, I don't worry. I've run more than one lockdown at 80-85% the whole way through to hatch, and in another I've had the humidity sitting up above 90% for well over a day. I've never had a chick drown yet.
The problem with humidity is that a lot of people don't actually understand how it works in relation to development and hatching. If you over-humidify your eggs from days 1-18, your chicks will in all likelihood still be alive by day 18. They're still enclosed in the membrane and not breathing air, so they physically cannot drown. Once they've been in lockdown for a couple of days, they pip internally into the air cell. This is when they first breathe air, and if your eggs haven't lost enough moisture, this is when they will drown.
So, people see chicks alive on entering lockdown, and dying during lockdown. Then they find excess fluid inside the eggs. So they blame the high lockdown humidity for the chicks' deaths, where in actual fact the real culprit is an over-high humidity the first 18 days of incubation.
Humidity is hard to get right and hard to advise on, as some people succeed with 30% and some people succeed with 50%. There isn't really a one-figure-fits-all answer as to what's the correct incubation humidity. MOST people would do well to start with 35% to 45% and fine tune it from there. But not everyone. I weigh my eggs to determine correct moisture loss and find that some eggs (usually ones from other people's birds) require higher/lower humidity than the 45% that my own eggs do well with. The benefit of weighing eggs is that you can adjust humidity as your incubation progresses and be fairly confident of getting to lockdown EVERY TIME with eggs that have lost the correct amount of moisture.
Hope that helps a bit...
