distance from canopy should be more about proper light distribution. use dimmer from there or hours of light.
assuming ambient co2, 35-40 DLI is what you want to shoot for.
DLI is best way to communciate this because it is the same regardless of any difference in garden size or hours of light. Daily Light Integral -- good thing to learn. (umol/s ppfd, par - good to know gist of these things)