Can't count the times I thought my light wasn't too bright and the plants told me otherwise. I'm amazed every single time at the progression of symptoms cause by too much light. Best advice one can give is get a light meter. I don't care what type as long as you use it as the benchmark for what works. Even a shitty one that only does lux/lumens can work as long as you use the readings as a guide to what works in your environment. It does not need to be a quantum sensor par meter, as long as the numbers you use are compared with the same light and same sensor it will be adequate to use as a benchmark for future use.
I tell people all the time to just use the free phone app. It may not be 100% accurate but it's definitely more accurate than eyeballing it. Especially with any phone that's been made in the last say 2 years or so, should be definitely accurate enough to go off of, as you said a benchmark
