Amen to that cuz I can't either . . . . just can't "see" it. The apparent difference is not the least bit subtle as it seems shockingly different (darker/lighter). Application of the principle seems clear enough, just check whether you've got it right or not. We should know after we've done it for it will nag at us that something is not quite right when its not glaringly off. We all have those experiences when we know something is not right but have no idea of what's wrong. I've ended up ruining an entire work because of a simple detail like this throws the whole thing off.
I just ran into a perfect example of this: working in white on black poster board, mixing black and white to get gray, it ended up looking brown instead. Now I have clouds with brown spots in them. Oh, well, must be air pollution.
@Mr.Magoo what kind of colors did you use? I do my black and white paintings only with Schmincke black. It has a warm and soft touch when I reduced it. The paintings become a nice "old" look. I never do mix it with white. I try everytime build up the grayscales with thin layers of reduced black.
I mean building up a tone of grey. When I spray one time a area I have a light grey. I repeat that so often I need and so Ican get darker and darker. The schmincke black is not a transparent color.