To calibrate your meter app, go outside on a clear day with full sun at it's highest point of the day. Measure the light. For me it was 94,000 lux. So, I know never to exceed that number. Ideally, I want about half of that at the top of my plants. The better fine tuned my system is, the higher I can go. 75% is pretty much the practical limit. This is based on my research and that of Dr. Bruce Bugbee. Take the same measurement every month and you will have the annual light curve for your area and you can mimic it at a lower level.
Yes and great hack... about half full sunlight is the absolute max without co2... wow great thinking brother takes almost all the issues out of using the app and not being able to verify against a meter. On clearest sunny day at high noon sunlight can be 130,000 but I'd say 100,000 ish is a good assumption.