I’ve found it to not be very accurate at all due to lights having different beam shapes, etc… It’s also not comparable to anyone else’s numbers as you have differences in table heights, ceiling textures, paint colors, etc… I never used a phone app, only a $30 Dr Meter LX1330B.
Basically you find a table or something similar and mark you a little spot to always tail stand your light. Then I mark a spot about 6 inches to one side to put the meter. Using the same light and meter orientation will give you repeatability.
Since light is additive, I turn off most of the room light except for a little lamp. I read this ambient level and later subtract it from the total. Then I turn on the flashlight and wait for it to either stabilize or hit the 30 second mark, whichever comes first.
I’ve never tried to equate it to lumens, but it does show me small changes in a particular light. Such as doing an emitter swap, spring bypass or a resistor mod, etc…
It’s not very good for comparing different styles of lights. Throwy beams and floody beams of the same lumens can give pretty different results due to the way the light reflects from your ceiling back to the meter.
If you have similar lights it can tell you which is brighter/dimmer etc…
It’s better than nothing.