Nearly everyone grows a bit too big at first. Supercrop, if it is an issue. You don't want to burn things. guessing about your lights -- that's REALLY CLOSE!!! lol... 18-24 is probably optimal, but could be wrong.
all lights are different.. .even if heat at 6" isn't a problem, number of photons (umol/s) could be... just depends.
If you don't get paling leaves that eventually bleach out in an interveinal way, the light isn't too strong.
If you don't have 'better' development under the canopy, that's another indicator you are okay. If trichomes are evaporating off the top, the lower nugs will look more crystalized. Development of buds should occur top-down, not bottom-up. That's a sign of a problem if it is bottom-up development. raise lights, if so.
i am only fairly sure you'll see some issues at that distance... wait and see, though.. first hint of leaves getting pale or crispiness, you better super crop your canopy so it's the distacen Mars recommends (check their website).