How easy or otherwise would it be to calculate how many of those selfscratches are only included by that DT1 user in question, for the full list of DT1s, the same as you have done for Dabs?
I did this by adding only one user to my alt's Trust list, so it's too much work to do. There are also cases such as Lafu (DT1), who's "Selfscratched users" didn't make it to DT2 because of DT1 exclusions.