The jury alone decides what is reasonable.The jury determines along with the judge what is reasonable and that is beyond the scope of the information presented here because so many of the facts are absent.
They do so on the basis of the law as it is explained to them, and evidence that is presented to them--evidence available from the scene, and other relevant testimony that may apply, but only that evidence that is admitted by the judge. Other things will not be known to them.
The jury may also base their determination on expert witnesses for one or both sides, when and if their testimony is admitted by the judge--expert testimony on things like reaction time, what can generally be considered deadly force, the impact of stress on perception, and so on.
Because the jury is to make its determination on the basis of what a reasonable person would have done in similar circumstances and knowing what the defendant knew at the time would have done, the judge will almost always refuse to admit evidence that the defendant cannot prove he knew at the time (the Tueller drill, for example, or the assailant's criminal record).
The judge acts as the gatekeeper regarding the admissibility of evidence, and he or she also explains the law, including relevant case law, to the jurors in his or her instructions to them. Those can include whether the defendant is permitted to claim self defense in the first place; when the use of force, deadly or non-deadly, is lawfully justified; such things as whether the failure to retreat in a state, in which there is no such requirement and where safe retreat was possible, may be considered by the jurors in weighing the question of reasonableness; and what can constitute provocation.
That list is not exhaustive.
With that single clarification, Post #75 is worth digesti--slowly and competely.