How do I change the default sort order of a custom column on the posts page?

I have a fancy custom column on the posts page in the WordPress admin. I have this column set up to be sortable. The first time that you click on the column header, the URL in the address bar changes to order=asc and then each time that you click on it, it toggles back and forth between asc and desc. However, I’d like the first click to make it desc and then begin toggling it. How can I do that.

Here is my current code:

So how do I make it so that when the column is first clicked, it goes to DESC first and then ASC? Thanks.