There is however conversions online from lux to PPFD. Then there are apps that actually measure light in lux with the front camera. There are different conversions depended on the fixture you use or use of sunlight, but full sun is considered around 108,000 lux. With my current set up, I have a lux of 25,000 from two different lights, which through calculations it ranges anywhere from 300-450 PPFD. Iebmug said before for full growth PPFD should be around 1600 (I believe full sun is around 1800PPFD). I'm finding it is super hard, and very expensive to replicate full sun.. it's better off to just allow dormancy almost unless you have the funds and a mini greenhouse to allow for optimum growth. However my question is, citrus are recommended to have 8-12 hours a day under full sun, so if I have lower PPFD lighting for longer periods of time, say 20 hours, or 18 hours, would that compensate some or would it further stunt the plant by not giving it a dark period?