The relative humidity in the incubator between setting and three days prior to hatching should remain at 58-60% or 84-86 degree F.When hatching, the humidity is increased to 65% relative humidity or more.
Humidity and Ventilation. The relative humidity of the air within an incubator for the first 18 days should be able 60 percent. During the last 3 days (the hatching period) the relative humidity should be nearer 65-70 percent. Too much moisture in the incubator prevents normal evaporation and results in a decreased hatch, but excessive moisture is seldom a problem in small incubators. Humidity is variable. The best humidity depends on your incubator and climate. The generally accepted is somewhere around 50-60% for 18days and then raise it on day 18 to 65-70%. Some do dry incubation down to as low as 30-40% and some have it as high as 85% when hatching. But my friend s making sure i know stuff before i atually incubate. Ive looked at quite a few sites and they say to incubate chicken eggs at 60 to 65% humidity for the first 18 days and then 19-21 up it to 65 to 70% humidity. when he asked what the humidity level needed to be i replied saying this and he said no. The humidity should be 70% at this point, and opening the incubator would lower the humidity. If 70% humidity is not maintained the membranes could dry out too soon and trap the chick inside the egg and not allow hatch.  The relative humidity in the incubator between setting and three days prior to hatching should remain at 58-60% or 84-86 degree F., wet-bulb. When hatching, the humidity is increased to 65% relative humidity or more. An excellent method to determine correct humidity is to candle the eggs at various stages of incubation. The normal size of the air cell after 7, 14, and 18 days of incubation for a chicken egg is shown. For still-air incubators, add 2-3 degrees F. 2 Measured as degrees F. using a wet-bulb thermometer. Use chart to convert to relative humidity. Sanitation In large commercial incubators, eggs of different ages are often set and each setting transferred to a separate unit prior to hatching. 1 If the incubator is not already in operation, start the incubator and allow the temperature and humidity to stabilize a day or two before setting eggs. 2  Set the temperature at 37.5°C (99.5°F) and relative humidity at 55% (84.5°F on wet bulb thermometer). 3  Set ventilation as recommended by the incubator manufacturer. 1 As the hatch nears completion gradually lower the temperature and humidity so that by the end of the hatch the temperature is at 36.1°C (97°F), and the humidity is at 70% (90°F wet bulb). 2  Vents should be opened to their maximum setting by the end of the hatch. NOTES: Humidity inside the incubator is affected by outdoor humidity. High/low outdoor humidity can raise/lower the internal incubator humidity.Hatch day can vary due to the age of the egg. It is common practice to set 5 to 7 day old eggs. Eggs older than 7 days may require extra time to hatch.www.GQFmfg.com GQF Manufacturing Company 912-236-0651.  1 At the time of transfer, the temperature of the hatcher should be set at 37.2°C (99°F) and the humidity set at 65% (88°F wet bulb). 2  As the hatch progresses, and eggs begin to pip, increase the humidity to 80% (93°F wet bulb), and increase ventilation openings by about 50%. 1 As the hatch nears completion gradually lower the temperature and humidity so that by the end of the hatch the temperature is at 36.1°C (97°F), and the humidity is at 70% (90°F wet bulb). 2  Vents should be opened to their maximum setting by the end of the hatch. This is critical on homemade incubators. It is possible to suffocate the eggs and chicks in air-tight container. The relative humidity of the air within an incubator for the first 18 days should be able 60 percent. During the last 3 days (the hatching period) the relative humidity should be nearer 65-70 percent. Too much moisture in the incubator prevents normal evaporation and results in a decreased hatch, but excessive moisture is seldom a problem in small incubators. The air cell increases in size during incubation at a rate that depends on temperature and humidity as moisture evaporates from the egg. The drawing shows the normal size of an egg's air cell at 7, 14, and 18 days of incubation. 
http://extension.psu.edu/animals/poultry/topics/general-educational-material/incubation/science-of-incubation/humidity-and-ventilation http://www.backyardchickens.com/t/173803/humidity-level-on-hatching-eggs http://web.uconn.edu/poultry/4-H%20Poultry/help_inset.html http://www.thepoultrysite.com/articles/166/care-and-incubation-of-hatching-eggs/ https://ahdc.vet.cornell.edu/Sects/duck/hatching.cfm http://www.poultrysupply.com/manuals/GQFEggTempChart_from_Kemps_Koops.pdf https://ahdc.vet.cornell.edu/Sects/duck/hatching.cfm http://extension.psu.edu/animals/poultry/topics/general-educational-material/incubation/science-of-incubation/humidity-and-ventilation http://www.extension.umn.edu/food/small-farms/livestock/poultry/hatching-and-brooding-small-numbers/
humidity chart for hatching eggs