Hey Growmie,
18 inches at 40% is a good starting point, Leaf color looks okay, though the newest growth seems a bit pale, which can also be due to low light intensity or slightly off nutrient balance (which isn’t uncommon this early). Try 16–17 inches from the canopy. Monitor the seedling for: Leaf curling or tacoing (too much light) Stretching (still not enough light) Burn or yellowing on leaf edges (too intense) Make small adjustments every 1–2 days based on how the plant responds. Also If you haven’t already, run a gentle breeze over the seedling with a small fan. This helps strengthen the stem and prevents it from falling over if it's stretching.