I don't know if I have a good approach, but I am not using these apps to get good readings. I'm looking for relative readings.
If I calibrate my app/phone to detect PPFD at what product reviews and/or the manufacturer rates the output, then I can get relative measurements at different heights and different locations in my tent. I do this to place my plants in the best places possible for my setup. It also helps me verify dim and height settings as performing how I would expect (again, in relative terms).
I don't expect perfection, but it's another guiding tool (in addition to back-of-the-hand, following manufacturer's recommendations--and, most of all, reading the plants).
Using PPFD Meter (the app) and a Verizon Samsung S8+, I have to calibrate down 11%. However, to get a 100,000 LUX reading in full sun (without a diffuser), I have to calibrate it down 65%. I've noticed that how I point my phone has a HUGE impact on the reading, so I try to point it at the center of my LED fixture (TSW2000).
When I calibrate to my light, I find that I get readings comparable to the footprint chart from the manufacturer. Therefore, I feel okay about using it to get general ideas of light intensity to provide another data point in my dim and distance settings and plant placement.
I would never make a decision based on a phone app light meter alone.
