Use your summary field and sort your records by CDC_NUM. But in the sort records dialog specify: "reorder based on summary field" and specify a descending order for your summary field.
That didn't work. Which, makes no sense to me.
I fixed the problem by moving the calculation into another table. I don't know why this works as opposed to what you suggested; however, I'm chalking it up to a hundred ways to skin a cat.
It does work. I've used it myself. You are telling FileMaker to do two sorts in from one use of the sort dialog.
The first sort is by specifying CDC_NUM as the sort field.
The second sort is the summary field that you specify in the "reorder by" part of the dialog.l