It’s probably like 20”.
I have a three sided box I slide in and line with that safety blanket to enclose the space when I’m not messing with them. I was really surprised how much that helps according to the light meter.
It helps a shit ton.
I saw some cheap car windshield reflectors, the ones you put in there windshield to keep the car cooler, and I've been using those for my seedlings. You can set them and box an area in to keep the light from being wasted.
I'd keep it at about 20 inches at about 60% and see how they do. If they seem to respond well, then keep it there for a bit. If growth seems to slow down and the leaves are droopy, you may be giving them too much. Of course make sure you are not over watering them too.
If they seem to be a little stretchy then you can turn it up more.