Yes it's the z steps per mm value in the firmware.
Problem is, it's got different names in different firmwares. Which firmware are you running, on which electronics?
PS: It's just a matter of changing the values, and then recompiling and re-uploading the firmware.
PPS: The value for Z steps per mm (and x and y as well) can be calculated. Prusa himself put an online calculator on the net that allows you to work out the proper values (for Prusa or any machine), but the link is dead (404) at the moment. I've passed on to him that it's broken, so hopefully it'll be back online soon.