@dcy For your general knowledge and awareness (but do yourself a favor and then FORGET about it for ordinary scene shooting!)
- the flat disk can be used for metering the illumination of FLAT ART
- the flat disk can be used for metering studio lighting to determine light source *intensity ratios* for portaiture illumination setup of lighting contrast
- the flat disk can be utilized for other techniques like 'duplex metering'
For usual scenes, the use of the incident meter is rather simple and free from error, when you
apply the very basics of use..
- put you meter in a area which is at your subject (or in light which iis very similar to that striking your main subject, when you cannot be at your subject)
- aim the meter hemisphere at the camera lens position, so it integrates what your lens sees, how the light strikes the subject
- make sure your meter is in the right mode (ambient vs flash)
any more is making it more complex than it needs to be ordinarily.
In comparison, reflected light metering is inherently complicated by meter angle of view, the error due to subjects themselves beeing brighter/darker than 'midtone average', all of which can entail the use of compensatory ways to counter-bias the errant reading.