A thorny issue. The accesability code requires the 80 inches. NFPA 72 gives the approved locations, as well as the manufacturers instructions. Note this is for strobes only, not smoke detectors. Smokes can be on the cieling. The best location for the strobes is 80 inches high. I believe there are some strobe units with increased candella output that are listed for cieling use on specific cieling heights. Check with the manufacturer.