50% is correct for the first 17 days Alison. On day 18 the humidity goes up to 75%, turning stops and the temperature goes down half a degree to 37.0C.
If the humidity is too high the air sac, created by evaporation of the water content in the yolk, doesn't develop fully. This then leaves insufficient space for the chick to get its head out from under the wing and break through the shell. You will need to check the sac development to the diagram I mentioned. We use what are called semi-automatic incubators. The humidity is controlled by adding water to troughs in the base, so its not exact at all. We check the air sac at day 8 and adjust accordingly. Our house is old and quite damp in Spring so in fact we haven't put any water in the machine until day 18. So to answer your question, chicks won't drown if the humidity is too high for a while, but it needs to be corrected within the same day. If it remains too high the result will be 'dead in shell', where the chick develops to maturity but can't manoeuvre to get out and dies in the process of trying. In the first 17 days you are better low than high.
I seem to remember a problem with the Janoil was that the eggs were not turned 90 degrees, but only 45 degrees. In some cases this resulted in chicks sticking to the sides of the shell. However a member in Northern Cyprus has just had a successful hatch with that make of machine, so perhaps my memory fails me?
