Here's how I do it, and no claim for the superiority of this method is made. It works for me...
I mount the gun and look over the bbl, noting that the beads do that Figure 8 thing. I then shift the focus of my eyes and attention to the front of the trap house. Someplace in here my left eye closes briefly as I call for the bird. 40 years of shooting means bad habits are very tough to break.
As I watch the bird climb, my gun follows it and when it feels right, the shot happens.It's a swing through, and the gun doesn't stop moving until the clay either breaks or leaves the area.