I have 2 tents, one for vegging and the other for flowering. I tend to keep my light pretty close to the canopy to prevent over stretching during veg, 12” at the least. I do however have a lot weaker of a light in my veg tent compared to my flower tent, but this is done intentionally because my vegging plants don’t need as strong of a light. Flowering is a totally different beast all together when it comes to PAR and light distance. As my plants grow during flower, I’m constantly adjust my light to maintain 18” at all times. But this is just the way I do things and it works out ok. I purposely force the stretch at the beginning of flower so I can train a little a better, and get the sides to stick out more because it’s searching for the light. So in the beginning my light is at 24”. Once the stretch stops I drop the light to 18” and keep it there till the end. I do however use LEDS, so I can’t say this process works with CMH or HPS though.