PPFD to measure light intensity

ppfd is a umol/s of light divided by meters squared. hours of light is factored with rate of light (umol/s). so you need both hours and umol/s to understand how much light you are giving a plant. it's about number of photons in various wavelengths over time. your temps and rh will be relevant too. CO2 will be the single biggest limiting factor on how much you can give. goe easy early. use the resulting internode length to adjust light intensity... not too stretchy. not too tight. early stretch is no big deal, especially if it'll be a big plant. it'll thicken up and the extra initial additional spacing can be a good thing.

You have ppfd meter for around 300£ but you dont have good light ? Otherwise stick to lux and lux app, dont put your nose were you can't sort it. Go simple, go with rules, when go manufacturer , then care about such things and tune up last few %

Seedling/clone 100-300 ppfd Veg 250-600 ppfd Flowering 500-1050 can go more if using c02 that's my general rule of thumb that I follow, hope that helps. Happy growing.