In order to arrive at Relative Humidity, you'll need 2 readings.
The first reading comes from your plain old Dry-Bulb Thermometer.
The second reading comes from your wet bulb thermometer.
Find the two values in their proper column, and read the relative humidity value where they intersect.
You now have your relative humidity reading.
Keeping proper humidity in your incubator will bring along your hatching skill by leaps and bounds. This chart will allow you to convert your readings into relative humidity. To find out the proper humidity for your eggs, see