My bator advises 60%. This depends on your location/climate. Im in Oklahoma. 55% to 60% is a good rule of thumb. Of you may and probably should use the 55% at 7 days from hatching. This works for me. You also shouldn't get it too low or the chick can be shrink wrapped. This works for me. Hope this helps.
Many people, especially those using styro incubators at normal elevations find that over 45%, especially 50-60% (with accurate checked hygrometers) will drown the majority of the chicks come hatch time. There are some that can successfully hatch at higher humidity. By successful, I mean at least 80% hatch from lockdown to hatching. If anyone is having multiple chicks making it through lockdown and dying at hatchtime, the first thing they should be looking at is humidity.
Humidity isn't a number you can just throw at someone. There are too many variables among hatchers, which is something these manuals and hatching resources do not take into consideration when they throw numbers or how many channels to fill at people. The best thing a new hatcher can do to get an idea of what generally works for them is to monitor the air cell for proper growth, or do the whole weighing thing.
Over 45% for me, generally and as an average will not allow for proper moisture loss and therefore will cause my chicks to drown. Standard sized, local eggs do best for me at 30-35% for the first 17 days to allow for proper air cell growth. Because I am a hands on hatcher, for hatch I up to 70-75%. My rates are generally 85-100%.
Because eggs vary in porosity, shell density/thickness, etc, it is the norm to have a couple that don't hatch. You can have a very porous egg loose too much moisture and a big thick shelled egg not loose enough in the same hatch. That is the reason you adjust to the majority, and become selective for setting so the eggs are of similar size and quality.