That's not true. The stress point is not genially set, it's a adapted and acclimated to. Most Indo-Pacific corals (where majority of the animals we keep are from) average about 82F during the summer and 78F during the winter. It's not uncommon to find corals in the low 90F and they have not shown to induct any thermal stress response. What we do know, however, is corals will start to stress roughly 3 to 4 degree above or below their acclimated temperature range. For example, if you keep your tank 78F constantly, they you should start to see corals stress below 75F or above 81F. If you keep your tank from 78F to 82F, the coral will response to stress below 75F and above 85F, etc. Within this range, you can fluctuate as much as want. Stability within the acclimated range is not important. The optimal temperature for calcification is observed to be roughly 2 to 3 degree below the maximum acclimated range. You stated that this number is 84F which is pretty accurate.