watch resulting internod lengths and adjust... i'd use height to optimize coverage and dim for early levels.. that way you dont' hve to keep messing with height. Distnce from canopy can make drastic differences (inverse square law applies)
your enviornmetn as well as genetics can make this different than other gardens. take notes and it'll be a good starting point next time while avoiding the trial and error.
QBs.. 18-24" or whatever best covers your garden. Use a free lux app and that will tell you proportional intensity.. measure center and edges/corners an dmake sure it's not too drastically different or expect much larfier buds along outskirts. probably 50-60% for early seedling but will quickly want more... avoid that stretch and don't stunt it either.
I would start with 35-40" above the canopy. If your leaves start pointing up( or young sprouts make a V shape then its to close. If the distance between branches are far apart(or seedling is longer then 3") then its to far. Every strain will react differently so there is no exact location for optimal growth. It will always depend on how your plants are growing.