The Ilford set I tested the gray stripe on the step wedge exposure was somewhere near the geometric center of the grays visible. I did not measure the exact reflection density.which midtone gray is supposedly to staying constant?
Well that is the tone you need to use to use the filter set correctly. If you try to judge exposure based on black, white or some other gray, you are not using the filter set properly as designed. I'd ignore criticism of a product what was used incorrectly.and second, is that always the right tone the exposure is judged by?