I'm just a beginner, so take it for what it's worth, but from my research, 76-78 is the ideal temp, but many have the mistaken notion that anything over this is "too high." That just doesn't make sense to me because pot grows well in many places that get well over 78 degrees and often never gets that low at all. I struggled with heat at first, with temps soaring to 104 (documented too high as the leaves near the light started curling upward) and never dropping below 90. After several adjustments and additions, I have got my heat down to a high average of 80-82 with lows about 75, and my plants are thriving. Is that "ideal"? No, but it doesn't seem to be "too high" either. After struggling with 100+ heat, I am thrilled with it, and my plants seem to be as well.
Meant to add that I leveled at about 88 for awhile, and my plants still showed minor signs of heat stress, so what Dizzle says above about over 85 starting issues seems about right to me. My plants were still growing at that temp, but they definitely like the low 80s better.