Red does new bulbs every grow. I had 75'sh % still left on a 6+ flower cycle bulb. When I replaced it , readings only went up about 20% with the new bulb.
Dirtbag was killing it with 2x 1000hps,,.
On a flat plane , more than 5x5 is stretching it.
I have 1400 over roughly 4x4, right now. I can average over 800 ppfd.
I have gotten satisfactory results with 2200 over my 8x5.
Soon I will be topping at 2800 ( 2x 1k , 2x 400 watt)
Regardless of light intensity, the thicker the soil the lower the yield. Not talking about quality here- just the contrast of hydro vs. Dirt growing. For some reason, in my head, I relate the total amount of wet and dry cycles directly to the overall mass of the harvest. Until we hear the sweet angels singing, we have to assume there is some factor limiting growth. With 1000 Watts over a four-by-four area, light intensity should not be the limiting factor for growth
Overlap is your friend. I would dim a bulb only if I ran out of Headroom. You can raise the fixture, in oder to reduce the brightest spot in the canopy to the maximum target intensity, whilst spilling light over to other plants.
Ps.
Do not put any stock in total PFD. Producing light is one thing ,,getting it to hit the plants evenly with point source illumination is another. A reflector can make or break a bulb.
I make evaluations and measurements with ppfd
That is a real-time measurement of photon flux density at one specific spot , at one specific time.
We map the intensity plotted over an area to determine the effectiveness of the unit.
The performance of the bulb is directly related to the quality of the reflector.