Simplicity is everything. Light is a different school of thought. For indoor growing if you're not adding Co2 a PAR of around 800 is sufficient, if you're using a PAR meter. You can get away with a Lux meter (30 euro about) with a reading of about 70,000 if you're using white LED's, so, full spectrum grow lights. For the blue-purple LEDs you have to use a PAR meter. Which is substantially more expensive than a lux meter, but you can get a cheap one for a couple hundred euros. If you're not at the stage where you want to buy a meter to measure the light. Just keep it at a distance so the plant doesn't bleach. That's the breakdown of chlorophyll which is the green pigment responsible for photosynthesis. If it's natural light, just leave it be.