PPFD is how much usable light hits the canopy (µmol/m²/s), so measure at canopy height, not at the lamp. Rough targets for photoperiod plants: seedlings 200-400, veg 400-600, flower 600-900 (up to ~1000 with good airflow and temps under 28°C). At week 3 aim for about 400-500. Photone gives a rough reading, a PAR meter is better. No meter? Use the manufacturer's PPFD map for your lamp at a given distance, and watch the plants: tacoing leaves and bleached tops mean too much, stretching means too little. Over a 18/6 cycle, 500 PPFD gives about 32 DLI