I have my focus target element moved to little right and down from the center. Enough away that I still see the center, but close at hand so it's on glance. On boss fights I focus the boss even though as tank I propably have it also targeted the whole time. I originally started this habit on healer as they don't target the boss and I still had to see their cast bars. Noticed it's easier to spot them on that placing on tank as well.