Overwatering doesn't mean too much water at one given feeding, it means watering too often before it's properly dried out and not allowing the soil to pull in oxygen.
Promix is nearly as impossible to overwater as coco, not quite but close. And it is VERY important that when you water, that you completely saturate the media beyond field capacity so that you force a little runoff. That runoff actually pulls a bit of oxygen into the media, and now the roots can fully explore the pot. The roots just won't grow into dry media, so you have to really wet it everytime you water.
What you want to be careful about is not watering again until the pot is quite dry. After transplant I'd expect it to take 5-6 days to dry out a 3 gal pot if the environment is correct, which it seems to be. Could be a little warmer really, LED benefits from temps closer to the 80-82f range to drive transportation. Those VPD charts are useless unless your taking leaf temps into account as the 3rd variable