Definitely you are not far from optimal position. I keep 2 of them on 75 cm from 4 modules and one of them on 55 cm, but she is not just under the light. The first two get some more light from lights afround so I keep this distance.
I monitor the stretch and go down with lights, but in small increments and slowly. Light down from 20 to 14 inches means exacty double energy for the plants. But it looks so little.
I like them to stretch a little bit in the first days (not to much, then they fall down), so that the first branches are not just 1 cm above soil. That is hard to maintain and water later. At least for me.
Well, and I am sure that the right distance between light and plants depends on how they grow and how you mantain them, too. So, what works for one is not necessary the best for another.