When, like me, you write the outline to force the story to cough up its entirety before you start the first draft -- much more economical than a stack of half-finished manuscripts, is a stack of half-finished outlines -- you have to learn the gentle art of how sketchy it can be.
Too much, and you've defeated half its purpose by going into too much detail before learning that it does indeed have an ending so the detail will not all go to waste.
Too little, and you haven't really determined that you've got a middle that will get you there, so you really haven't got an ending. It's one thing to know that the the heroine must separate from the others with her so she can meet the hero alone. It's another things to have sufficient reason for her to do so.
sigh. The gentle art of writing.