In the tests I've done it works best if it's above the Adam's apple, as close as possible to the jaw and with the head turned a little downwards. My vision starts to go dark in just 2 seconds, simply pulling the rope with my hand.
You need to test this out before doing the ctb attempt, if it's in the correct location the vision should go dark quickly, otherwise it's in the wrong location and it will take time to pass out on the ctb.
Very careful when testing this, use your hand to pull the rope and not your body weight.