One way to go about this would be to guess what the DBH would be by using the stump measurment and a comparison with nearby standing trees of the same species and similiar size. Then use a scale to get bdft volume.
It would have been much more accurate and simple to have done this before the trees were cut.
Reading between the lines, I get the feeling that you didn't know the trees were going to be cut. If that's the case, you probably need to hire a professional forester to appraise the missing trees. He would have more credibility legally.
Noble