i was at that height 18" at those levels for seedlings, plants have since grown and are in that 8-12" range from the light but i have yet to really turn the light back up as im now cautious.
Also consider now that were getting better and better efficiencies out of lights but yet we still want to throw as much light as possible at our plants so a metric of 40-50w/sqft shouldn’t and doesn’t really apply anymore yet we still do it.
Im guilty here too cause i designed my lights to run around 120w max which would be around 40w/sqft, but im currently playing in the 65-75w range, oh well lol