How large is your system?
For my tank i started very conservatively @ 10mls per hour then just kept bumping it up....
I currently go by the ACI Chris Meckley method. Basically, you are just replacing your daily evaporation overnight. So, if you have an ATO you will take this offline and will have to figure out just how much water you are evaporating daily. Right now, I'm dosing 1900mls every night. I am also forcing some evap. with a small fan on my sump. Mainly doing this to help keep pH up since i am in a high CO2 environment and pH drops quickly. Honestly i stopped caring too much about an exact number for KH... mine is sitting at 12.1 according to salifert test kits and my tank has never looked better at 2 years old currently. Other factors play into this like light, nutrients and flow.
hope this helps.