I go by light intensity, not inches. Personally I use the Photone app on my iPhone to measure the light intensity and for flower I keep it between 1300 - 1800 ppfd depending on genetics. As George Cervantes put it back in the 70's, "lumens are for humans." Our eyes suck at determining light intensity, I prefer to let computers do the math.
The height I put the light at depends on the temperature and humidity of the room. In the S. US it's mostly hot and wet year round, so I tend to keep the lights as far away as I can while keeping the intensity up, that way the heat burns off some of the humidity and the moisture is under control; as long as my temps stay in check. During the 2-3 low humidity months I might drop the light more and lower the intensity if environmental numbers are going well.
I usually give the plants a 3 day - 1 week adjustment period to being put into flower. Then I blast them with the max PPFD they can take until the last week of flower. So if it' s a 9 week flower strain, week 1 will be at "veg" intensity of about 800 - 1000 ppfd, then the middle 7 weeks as high as the plants will take, and then half a week to week back at around 900 ppfd.
That's what I do, your mileage may vary. I've seen some LED model lights that simply cannot burn the plants, where people have grown straight into the diodes. A few years ago I found a Youtube kid who put 1 QB288 on a 600 watt driver and pushed it at full power until the diodes start popping. Even with 3 boards on a 330w driver, they're slightly overpowered even if they could take more electricity before blowing diodes out.
It took me about a year to a get the system tuned, the best advice I can give you is to meter your light with a measurement device of some kind. A $30 par meter from Lowes, a $20 phone app, or a $600 quantum sensor; it doesn't really matter as long as it gives you the same number on Monday and Friday if you have the sensor and lights exactly the same. Get a benchmark that is consistent and then check against your plants if it's too much or too little. There is no single number for a light or a genetic. I've run 5 different genetics under this light and every one has liked slightly different intensities.
With dimming and LED's, IMO distance means nothing. I can turn my veg driver down to 30% and get 200 ppfd at 6", or I can move them up to 42" and turn the electricity up to where it matches the intensity where it was at 3", but it's a waste of electricity and creates too much heat. It's a balance of power usage, heat, and intensity. Light distance and dimming amounts are just adjustments to control those parameters.