Bill Tindall, E.Tn.
On posts like this one I often seem to be the only one that doesn't understand the question. Does this dovetailed piece hold the case sides together like the drawer dividers show in my attached picture? If so you are proceeding backwards. Once I am sure what the need is I can answer the question.
Will this joint be visible in the finished piece?
What is FWW ref. I have them and can look up this project.
I don't know why people use double tails for holding sides together. Vastly easier to use a single sided tail for this task.