We do not have any specific engineering recommendations for installing a vent hood over a kiln. However, as a general principle you will want to have the vent hood as close to the kiln as practical to maximize the focus of the vent suction. The countervailing factor is the height of the kiln lid when raised and any ergonomic factors such as does the vent hood present a bump hazard for personnel. A typical installation for a 27" high kiln might be a little over 6 feet from the floor.
In addition to a hood vent you may also want to consider putting in one of our Vent-Sure Downdraft Vent systems and expelling the output of that into the hood.