This has already been answered. I usually hatch mine at 65% (though if you fill the all the water tray on the genesis, it will bring the humidity to exactly 75%, which has worked too).
I really think the key is in not opening the Bator. If you run a 50% and do not open te Bator once, you still will have enough humidity for the chicks to hatch with out problems. If you run a 50% and open the Bator for any reason after eggs have externally pipped, you have a high chance of getting glued chicks. I remember talking to some people about the difference between shrink wrapped and sticky(I call tem glued) chicks.
Shrink wrapped is caused by low humidity for a duration of incubation, causing moisture loss in the membrane. High humidity can help at hatch, but the biggest problem is the ability for te chick to move in the membrane.
Sticky (what I called glued) is low humidity (or draft of low moisture air from opening Bator) after external pip. This causes the normal papery membrane that should be in the egg to get glue like and stick to the chick (stick to anything!) making movement darned hard for the chick to move and properly escape the shell. If you have a seriously glued chick you may have to assist, though last resort. A spritz with a sanitized water bottle can help the egg and the incubator if you open it. I had to help a sebright a few days ago. It pipped in my incubator score I moved it to the hatcher. The time and change it took to move the egg from the Bator to hatcher caused bad glued membranes. Luckily, I didn't have any other pips so I could help it with out worrying about getting into the Bator.
Though I don't recommend it for those who aren't familiar with hatching, I have found running a 75% humidity can allow quick peeks into the hatcher to make changes or grab a bunch of chicks if they are being to rough with the other eggs. 75% seems high enough that the humidity doesn't drop down to levels that cause stickiness, if done quickly.
I have only hatched duck eggs once (never again, I can't stand the mess ducklings make) but mine took a LONG time, compared to chickens, to go from iternal pip to hatch. Mine peeped, and I think it took three days to hatch after I started noticing peeps and internal pips. I really thought they weren't going to make it, but 10/11 hatch just fine!
