You want 400 ppfd in this stage (up to 600 ppfd with CO2). Look up a "ppfd map" of your light. Put it at the recommended level and leave it.
You can save a lot of electricity by dimming it but you need a lux meter at minimum or a PAR meter if costs don't bother you. Without measuring the lower power somehow, you won't know if you're getting the right amount of light to your babies.