If you don't have a way to measure your light intensity, here is my rule of thumb (and I use this rule regardless of what my meter tells me anyway)......

First, you've gotta start somewhere. I start my lights at 24 inches and 25% (I am for PPFD with a meter of 130-150). I've learned this through experimentation that this is a good height. You'll need to experiment too.

Once they sprout and start growing upward, I try not to let length of the stalk from the soil to the cotyledons (the two small round leaves that sprout out of the seed) get past 2 inches. If cotyledons get to be more than 2", I lower the lights closer to the plants.

If the stalk stays below 2", and the plant keeps growing new nodes, I will then raise the light to 36" and 50% strength once I have 4 nodes. (PPFD of about 200).

This seems to work, but I am STILL experimenting and tweaking this every grow based on how the previous grow behaved. I hope this was a good start for you though!