How close should my 300 watt led be to 6 day seedlings
Not nearly enough info to tell you. Need make and model of light for starters.
If the manufacturer has produced a “map” of PPFD readings at various heights you can use that as a guide. For seedlings the PPFD should be between 200-300.
If you have a quantum board led light you can use an inexpensive phone app and get accurate enough readings so you don’t have to guess. If a red/blue led the phone apps aren’t as accurate but still might be better then guessing,
Failing all that, start with the light up high and /or light dimmed way down, then watch the growth over a weeks time. If the seedlings get too long and stretchy you need to give them more light. If the plants struggle with burned tips or compact growth you may need to dim it more. Trial and error.