The NRA recommends the ol’ half exhale trigger timing. Let half of the air out of your lungs, hold, fire.
This technique works just fine, though we’re partial to the full exhale, “down breath” concept. In the down breath, you draw a deep breath, exhale completely, then fire in the pause before inhaling.
Whichever you choose, the important thing is that it’s comfortable to you. Try both and see what works. Once you’ve settled on a breath control technique, you’re in business: Controlling your breath.