I mean it. Do NOT change one word in your final act until you have done three things–and I mean ACED them, not half-assed! What are they?
1) Your query letter, whether this book is already sold or not.
2) Your synopsis, one page per 20,000 words, three pages max, PLUS a shorter, one-page version.
Why? Because you want this book to rock, because you want your readers satisfied and because I TOLD YOU to. Follow orders!
The fact is that nothing will catch your plot holes and help you gauge the strength of your ending better than a synopsis. The query helps you figure out if you have fully realized the initial hook. Keep in mind that each of these docs has its own goal. Each should be fun and exciting to read on its own.
I recommend the resources at Absolute Write Water Cooler’s Query LetterHell for help on both. You’ll need to join the free forum, then go into Share Your Work and Query room to find the very best resources. These things are HARD to write but will teach you so much about your book–and its soul.
3) Before you go hacking up a third act that should FLOW like a raging stream, set aside your manuscript for a week. Don’t even peek at it. Then read your damn book. Start to finish. Don’t change anything, don’t fix a comma. Experience it as a reader.
Only then will you be ready to revise your final window and Act 3.