If you are going to use a handheld light meter (dedicated or phone ap) you need to know HOW to use it.
A traditional light meter, hand held or in camera, is a reflected meter. It measures the light
reflected from the subject/scene.
The TTL matrix meter is a specialized type of reflected meter, but still a reflected meter.
The cell phone and light meter app is a reflected meter.
The other type of hand held light meter is an incident meter. It measures the light
falling on the subject/scene.
Many reflected meters could be converted to incident mode, by the use of a correct white dome over the sensor.
Each type has its own pros/cons and limitations based on how they work.
Whichever type you use, you have to know how to use it, within its limitations.
Personally, I am an old incident meter guy. I used incident for a LOT of slide photos back in the film days, and I occasionally still do.
But there were times when I could not use incident, and had to use reflected; recognizing the limitations of the incident meter.
I also have a 1 degree spot meter for measuring reflected scene brightness, for my 4x5.
So as far as I am concerned, there is no ONE perfect meter, yet. OK, the modern Sekonics are close, combining reflected, incident, spot and flash, into one meter.