I always use the top to find center when I'm looking for CTE. Bottom is less objective, especially with varying light conditions.
A couple of things I've picked up that help acquire the perception more quickly. One is to work harder at being close as possible on the alignment I'll need to make the shot before I even start looking for the visual. The other is to first look for overlap. For the 15 degree perception, I look for 3/4 ball overlap and then look for CTE, ET A or B. The combination of the correct alignment and 3/4 ball overlap get you very close such that slight head movement finds CTE and ET A or B very easily and quickly. I look for 1/2 ball on CTE, ETB. 1/4 ball for 45. 1/8 for 60. At least for me, finding the alignment and visual for 1/8 is the toughest. Thankfully, these shots don't come up as often but when they do, you need to have practiced them so you're ready to make the shot in competition.