Phone app PPFD meters can work decently, I use one in my own grow to confirm hanging height, and reference it to the manufacturer stated PPFD for certain hanging heights (MIGROW is good about providing that information.) Your 46 PPFD for the 240 watt LED seems unbelievably low so I would look into that. Make sure the app adjusts for the correct light spectrum/colour of your particular light (eg. 4000k) and keep in mind your phone will measure from the camera on the some side as your screen, and not the one on the back. 200 PPFD for a young seedling, 400-500 for veg, and upwards of 700 in flower are the numbers I aim for. Happy growing!