This spot here, there's a top directly under a vertical hood. 12 inches (ish). That's about as close as I've gotten without seeing any ill effect.
This room is a hodge podge of different shaped 315 hoods, and HLG boards.
Took it right to the brink of overgrown!!!!
Also to be noted, this has been flowering at 10/14 for just beyond 30 nights. The photon saturation is so high, I can get away with shorter "days".
I generally have a little more wiggle than a foot, but they seem to be taking it well.
@Skunky Dunk Farms
, I also use a cheapo brand(Dr. Meter) LUX device.
Works wonders with white LED/CMH to get an idea of light levels. Not nearly as precise as an actual PAR meter, but once I know how my meter reacts to my equipment, it's a great tool for baseline setup.
30K and above LUX, pushes chunks.