If I go any closer to the lights, they actually get less light because they end up in between the individual tubes. Furthermore, the ambient temperature in that room is in the low to mid 60s, but the temperature 3 inches under the light is close to 90.
When I grew my onions, they didn’t seem to mind the light at all. Nice, sturdy plants. As far as watering, I’m concerned about under watering, because when I grew the onions, the media would be moist in the morning, and I come home from work to wilted plants. It seems like they go from “too wet” to “too dry” very very quickly.