Generally good Light Levels (PPFD)
- Clone stage: 150-250 PPFD
- Vegetative stage: 400-600 PPFD
- Flowering stage: 800-1200 PPFD
Depends on the light and environment of your room, if the room is hotter (say 82+) I have noticed I need to raise my lights even if they are dimmed all the way at times. Primarily in veg as my flower room stays around 75-80.
I wouldn't worry much about light distance and worry more about PPFD and how close you can get the light to the plant before you see stress signs. Once you get a "feel" for it you will be able to dial in any light system without meters or measurements just by moving lights based on experience.