Hello there. I just got some really useful information about this! 
What you want is to lose between 14 and 20% of the moisture in the egg to create an adequate air chamber in the egg. Depending upon the ambient humidity where you live, it may take lowering or increasing the humidity in the incubator to facilitate this loss. If your incubator lacks airflow, you’ll want your humidity a little lower. If you live in a dry environment, you’ll want to prevent too much water loss, and you’ll want your incubator set to higher humidity.
Living in Hawaii, sometimes our weather creates high humidity. It did this with my first hatching. Our humidity was at 80%+ at times. Partly due to this, I had a 75% loss. I thought it was soley because the eggs had sat out too long before incubation. Looking back with what I know now, it was clear that there wasn’t adequate moisture loss to facilitate enough airspace in many of the eggs at the high end of the typical, recommended humidity. 
My situation is a bit extreme, but a good illustration of the concept.  I am now keeping my incubator at 40%, as well as selecting out any flaws I see on the shells to optimize my hatch rates.
What were your results?