Novel Progress: Writing on a 'Need to Know' Basis

Hey, Blog Buds.  Check out my wordcount thingy to the right, slowly but surely.  The funny thing is I keep bumping and weeding and the starting point, namely the first three scenes.  The first scene, the first chapter, and the first three chapters of a novel are the most challenging, followed be dead center and the finale in that order, at least for me. 
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There's a saying about starting a story at the point on the fictional timeline in which the Hero or Heroine 'starts a fire she can't put out.'  A novel has lots of fires, so this can be hard.  That's when a girl has to go back to theme and main plot.  So, all the scenes I've written are good and fine, but figuring out what order to put them in takes Trial & Error.  I'm constantly asking myself 'Does the Reader Need to Know this yet?"  If not, it can be put off.  So, I switch things around again until the right fire rises to the top.
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I think it may be extra tough because this is the final book in the series.  Everybody and everything must come full circle, loose ends need to be tied up.  Think about Harry Potter and the Deathly Hallows, Part Two when Professor McGonagall suggested asking Seamus for help blowing up the bridge.  If you've watched and loved all the movies, you know Seamus is famous (hey, that rhymed) for having his spells explode from his very first year at Hogwarts.  When little things like that come back by the end, it's a wonderful, nostalgic feeling.  So, I'm going to mine the first three books of my series for little things like that too.
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Okay, I must get on with my day.