John....Try to think of it as...a simplifed version in the form of....
Teaser should be the Beginning / Tee up
Middle should be the action towards the conclusion
End should be the wrap up from everything before.
I am of the understanding for a TV episode for TOS Trek that it should slice loosely this way...
Act 1...beginning + Middle
Act 3...middle + lead in to Conclusion / ending
Act 4...wrap of of middle and ending
Just evenly break the 3 part ( beginning / middle / end ) up evenly whichever format you are working with. There should be some flexiblity but should tack with this as a general loose rule of thumb.