Hey Growmie ...
When they are seedlings 30" would be perfect but once they are in veg and flower you'll need to move the light to 18"-24" away from the canopy. You'll know if its too much light because the plant will start showing signs of stress, and it doesn't then you leave it at that distance.
Hopefully, this was helpful and remember ..."Happy Growing" and "Free your mind one puff at a time"
Mr_Weeds_Autos.