Casey: I've done what Joe suggests with my birds at various times - usually shortly after each of them came to live with me. The method does work to teach Macaws where the maximum acceptable finger-squeezing pressure threshold should be. I have in fact used this method on Salsa. When he arrived here (over 5 months ago), he liked to use too much squeeze pressure when beaking my fingers. But he has learned through this method where the threshold lies. He now mostly abides by this awareness.
The way I perform the beak separation method is by placing my middle finger inside the lower beak (i.e., below the bird's tongue, but resting my finger on the bird's inner hard beak surface rather than back into the soft tissue area near the under part of the tongue). I then pinch my thumb and my index finger together at the tips and rest them against the underside of the bird's upper beak surface. This allows me to spread my thumb upward and away from my middle finger and thus pry the two beak halves apart as the Macaw tries to squeeze my fingers beyond an acceptable pressure point. It also allows me to avoid making contact with the forward cutting edge of the bird's lower beak. The method will require multiple repetitions and consistency. But large Macaws will learn from this method if you maintain that consistency.
(Note: The above is simply one method to convey a finger-squeezing threshold to a large Macaw. But, again, it's a method I've had success with. As has Joe, apparently. However, be mindful that other methods exist as well.)
Time outs last about 5 minutes, or more, if I see his still has a case of AGITA. 
