If a situation requires immediate deadly force to prevent great bodily harm or injury, than deadly force will be immediately applied.
I have no intention of ever "threatening" to use deadly force, as it is either justified, or not, depending on the circumstances and state law. Even in non-castle doctrine states, citizens are not bound to a comprehensive escalation of force matrix as are LEO's or military.
Know your laws, and be able to justify your use of force if necessary.