ES 300 V2 lights - how to use the dimmer correctly?

I'm on Day 26 from germination. Plants look fine and healthy, no issues there. I've been using two T5's, 6500 each, since the beginning and I'm going to set up the closet this weekend to move all four plants. I just found a local source for lightly used ES 300 V2 lights and the price was too good to pass up.

I've only ever used T5's for veggies and now this, so I have no experience with LED's.

I'm still in veg for another couple of weeks (autos). My questions are re: how far to space the light from the plants, and how to use (or not use) the dimmer function on veg vs flower? Currently have the timer set to 18/6 with the T5's. Moving to LED should I mess with the timer as well?

Thanks in advance for any help. This community has really helped me gain confidence and I'm always open to learning from folks with more experience.

Here are some PAR readings at different heights. That'll give you a good indication on what height to hang it.