It’s very hard to judge this because every light is different. Take for instance, I have a 250 Watt light and another 700 watt light. If I put my 250 watt light 18 inches above and turn it to 30% the light is going to be much less then the 700 watt light turned to 30%.
It’s best to download a PPFD App and measure your light at the top of your plant. For seedlings, a PPFD around 300 is great at 18 hrs on and 6 hours off. If you do not have the ability to download a PPFD app, the manufacture has suggest recommendations for the light in which they tested all the PPFD levels for the various stages of your plants growth.
I hope this helps.