There's another issue, as 5600 feet corresponds to 2.5C per thousand feet. The rule I remember is that the difference between temp and dew point drops by (on average) 1C per 400 feet, i.e. 2.5C per thousand. Temp drops by an average of 3C per thousand feet (rising unsaturated air), but dew point also drops by 0.5C. Something to do with density, but I can't remember exactly.
Edit: I found this reference in
http://www.abc15.com/content/weather...cloudbase.aspx :
Dew Point, along with Temperature, can tell you the estimated height of a cloud's base. How? When air rises in a convective current, it cools at the rate of 5.4° F [3C] /1,000 ft, and its dew point decreases 1° F [0.55C] /1,000 ft. Therefore, the temperature and dew point are converging at 4.4° F [2.5C] /1,000 ft. Since clouds form when the temperature/dew point spread is 0°, you can use that information to estimate the base of a cumulus cloud. Here's how it works: Subtract the surface dew point from the surface temperature to give you the temperature/dew point spread. Then divide that number by 4.4° F. Finally, multiply this result by 1,000 ft. This total will be your estimate of the cloud base's height above ground level (AGL).
Or, multiply temp / dew point diff in deg C by 400, to give cloud base in feet.