Something a lot of people struggle with when calibrating the light meter is the calibration… on this blog I have a special selection on light meters (check it out :-)).
When I teach my students to calibrate the meter to the camera I always use an 18% gray card and shoot this in a as flat as possible light situation and make sure that in the workflow I use the values for this card are 128.128.128
Here is where the confusion starts.
Some cameras are calibrated different, cameras use reflective metering and are set in values between 12-18% gray. Meaning that some cameras will yield different exposures, which can be compensated. A lot of this has to do with the gamma curves and different colorspaces. For example when we look at LAB a gamma curve of around 2.47 will Yield a 128.128.128 value, but mostly gammas of 1.8 and 2.2 are used in colorspaces like sRGB and ProPhotoRGB.