No, the 3 rules are generally all that apply:
-the person must have the ability to inflict severe harm or death upon you
-the person must appear to have the intent to inflict severe harm or death upon you
-there must be no way for you to retreat/avoid the situation (except in your home, business, fenced in area of property)
Disclaimer:
In many states, you may intervene when a 3rd party is threatened in that manner- putting yourself in their shoes to gauge your course of action. In some states you may use lethal force against many felonies that might not necessarily involve an unavoidable threat to your life, such as commonlaw burglary. In some other states, you must retreat from your home. In some other states, great bodily harm is not sufficient- it must be immediate threat of loss of life with no path of retreat. In some countries, you cant even defend your life.
If the confrontation hadnt reached a point justifying self defense before you drew your gun, you are in the wrong for brandishing to escalate the situation. Thus it would clearly be a crime of some sort depending on the reasoning and mood of the DA or cops who show up.
If the confrontation has reached that point, you are justified in using self defense and thus justified in shooting, regardless of whether you warn that the device in your hand is a loaded gun that you intend to use.