Another way of doing this is to meter an object without the filter...(the sky for instance) then meter through the filter on the same object. I do this when I use reflective spot metering. Obviously when I use incident metering then I use filter factors.

The reason that this will enable you to more accurately determine the effects of the filter is that "filter factors" do not have the same effect on different colored objects. In other words a #25 Red may have an 8 (3 stops) effect on a blue sky...but it would have a far different effect on a red fence, for instance. It would have another effect entirely on green foliage.

A yellow number 12 will have one effect on the sky and another effect on yellow straw.

That is why when one is attempting to visualize how a scene will render the information gained by metering without and then with the filter is very informative.

A study of a color wheel will help in understanding the effects of filters...same colors lighten...opposite colors darken.