With a PPFD at 300 you well only get a DLI (Daily Light Integral) of 25,9 if you run your lamp 24/7.
From what i know seedlings/you plants need a DLI of about 20, so you should be good but i would aim for a DLI of at least 30 even for small plants.
Par is lights output, wavelengths and such area covered and so on
What you're needing to understand is ppf/ppfd.
4-450ppfd for a young veggin plant.
Have a read on growweedeasy to understand. Ill try find a link and dm