Datasheet here : http://www.sparkfun.com/datasheets/Sensors/Weather/SEN-09569-HIH-4030-datasheet.pdf

Maybe I'm not understanding how to use the analog voltage reading from the Arduino with this sensor.

If 0 is 0 volts, and 1023 is 5 volts, it looks to me like this sensor would output a value between 0 and 765 from analogRead()

I'm getting that 765 number by taking the maximum voltage output according to the datasheet (3.75 volts at 25 degrees C and 100%ish RH on average), and dividing that my the 4.9mV per unit that the analogRead function is supposed to return.. So :

3750 / 4.9 = 765.30

If 765.30 is the max the sensor should return, then getting the RH% *should* be to divide the value from analogRead() by 765, and multiply by 100, correct?

It can't be, so I'm clearly doing something wrong ;-)

I have this setup in my office and the average reading is 265, which by my calculation would me 34.6% RH - but that seems REALLY low. Unfortunately I don't have any way to verify that number. Can someone help?