And if it's not difficult, tell me how I shou...

Too many relativistic factors.. so no matter what you read or hear, you still need to observe and react to the plant... take some notes per season in your region as to what works best. Even genetics add some variance to this. QBs are better 20-24" away in mature vege and bloom, imo... you need the distance to reduce hotspot in center and allow for distribution of photons emitted to reach sides/corners. You may want to raise it further for yongn plants or make use of dimming... if you have a ton of plants and large area to cover i recommend raising the light... otherwise dimming and lowering for a smaller front print can save some money and won't make a difference with young plants. in mature vege or bloom, i always prefer using height from canopy to manipulate light intensity rather than dimming. hours per day is another way to keep the coverage more even while avoiding light stress without dimming. More photons vs fewer is a good thing. the corners and sides will appreciate it. trial and error... watch internode length and general health of plant... it'll quickly tell you if it is too much or too little by teh amount of stretching occurring. If leaves from growth nodes stack up on each other, needs less light. if it gets too leggy, it needs more. Take a few notes.. if ayou have seasonal changes might want to re-verify under different environmental conditions. light, rh, co2, genetics, nute mix all impact how well you can make use of light and therefore how much per day the plant can handle without a problem rising up (in either direction)

Hello there. Download an app on your phone called photone. You use the front facing camera to check the strength of the light at the canopy. Start off at about 10DLI then increase the strength a bit each week until you hit about 40DLI around week 5 or 6 and that should be about as strong as you need. You will be able to tell if the plant wants a bit more light and adjust either the driver power or the light distance.