It is up to the prosecutor to prove, the defense to refute. Postmarked envelopes may help to refute... but they are refuting a red herring argument.
The standard (ignoring statutory standards like "vandal after dark in Tejas") is "reasonable person", not "educated person". Humans are well known as inventive and problem-solving entities... a reasonable person, when faced with a threat, can potentially work out the likelihood of danger without referencing any empirical studies or educational materials. Ergo there is no need to prove that you were following your training as a justification for your actions.
If you can produce an expert witness that says, "I've trained countless police officers that any armed person within about 20 feet can pose a mortal threat", you have demonstrated that your actions were those of a reasonable person whether or not you took those actions because of prior training.
As such it really doesn't matter when the prosecutor says you learned something unless (s)he can conclusively (beyond a reasonable doubt) prove that you learned it after the incident and are claiming otherwise. In that case they can use it to discredit your testimony. That weakens your position but may or may not prove, beyond a reasonable doubt, that you were doing something more than defending yourself.
The flip side is that training is not a substitute for judgment. You've got to apply reasonable standards that fit the actual circumstances no matter what, and how, you have been trained. You can't just say, "I was trained to do this in the army and so I applied that training", for example, because some army training is unreasonable in civilian circumstances.