I know the law well enough to be certain of its application, given a supposed fact pattern.
If Zimmerman is retreating, has broken off searching for Trayvon, then Zimmerman is not in the act of closing distance between himself and Martin. He's not looking for any sort of confrontation - not a verbal one, not a physical one.
There was a physical altercation. I find the available evidence to have Martin the aggressor. The 911 call has Zimmerman screaming for help, so it seems he doesn't have the option of escape.
The question at that point isn't whether or not Zimmerman exhausted means of escape. The justification of use of deadly force depends on reasonably apprehending serious injury or death. Being beaten with fists by a person who has overwhelmed you with physical force EASILY meets that threshold.
-- If Zimmerman provoked the use of force by Martin, then he could not have used deadly force unless he had exhausted every means to escape the danger other than the use of deadly force. --
A number of posters have the impression that following a person is adequate provocation, under the law, to justify use of physical force. It's not. All the indicators point away from Zimmerman provoking or initiating force.
If Martin was afraid, and had eluded Zimmerman, then Martin's best action is to get in the house where he is staying - all the while avoiding Zimmerman. And if Zimmerman comes into view, all Martin has to do is state "I'm staying at xxx with my dad.", or even a less cordial, "buzz off, bitch. What I'm doing here is none of your business." I don't see Zimmerman, who is armed, risking a physical altercation at his own initiative. He'd be the aggressor, lost the right of self defense, if things get bad he'd end up on a manslaughter charge. His CCW training taught him that.
. . . and that was the purpose of linking the Florida law here: so others can apply the actual Florida law to fact patterns.