How do I regulate humidity in my incubator?

Hi guys! I have a still air incubator that I like to try to hatch a few Silkies in each year. I just don't know how to measure/regulate the humidity. I have an egg turner so I just squeeze in a cup of water and hope that it works. I don't have great hatch rates though (although somehow I did manage to hatch a duckling last year...lol). They start growing and then die before they get much bigger then my pinky fingernail.
Is there a humidity thermometer or something? It is a Little Giant incubator, and it looks like this :
http://www.flemingoutdoors.com/little-giant--9200-still-air-incub.html

Thanks!