You could try this:
Put your camera in manual mode.
Find a scene, set the adjustments to proper exposure (the middle)
Screw on the filter, and note the difference in the camera's meter. Also, if using a DSLR, keep in mind to cover the eyepiece of the camera as this will allow light in and screw with the exposure meter.
The problem with a light meter is that you have to tell it how dark the filter is so that it can compensate. Or you can dial it in using an app.
I don't have a variable ND filter, but if the filter is marked with this information, it shouldn't be a problem.
However if the filter isn't marked, it can be tricky.
Fixed ND filters could be easier to use.