I agree and disagree. Yes you have to assume that the historically optimal exit point remains optimal but that is true about all aspect of your trading.Quote from AAAintheBeltway:
While I tend to agree with you conceptually, I think there is a statistical flaw in the argument. You must assume that the historically optimal exit point remains optimal into the future.
The strategy you use is based on its history and you assume it will keep working. How do you know that any aspect of your strategy is still valid going forward? You don't - that's what trading is all about.
To say that you are not going stick with the historical optimal exit point because you think it may no longer be valid means that you dont trust your system anymore. If 6 used to be optimal but you are not sure it still is then why is 3 a better guess? Surely you need to have some rational reason behind such a change.
If your post-trade analysis uncovers that 5 is now a better exit then 6 then by all means change you exit from 6 to 5. I just dont ever see a reason to exit half at 5 and half at 6 - that sounds like an emotional decision, not a logical one.