I want to place an pre order for my mavic. I am hoping to have it by January.

If I make a payment now, when is the latest I can do a cancellation and get my full refund, if the Mavic delivery is later than I have been promised?

Right now, the last statement that I saw was that they hope to have all orders (from the DJI online store) cleared out by the end of the year IF they were ordered Nov. 3 or before.

My recommendation: Try local hobby stores and get on a few of their waitlists. Also, OBSESSIVELY check in with local Apple stores in person and ask them to check physical stock - this worked for me! (It's hit or miss because you basically have to get them RIGHT after they've gotten a delivery but before online stock is updated otherwise someone will purchase it VERY quickly). NewEgg or Amazon may also be decent bets. Avoid B&H and Best Buy is a bit of a mystery at the moment. Generally, I'd avoid the DJI online store like the plague. Look through the forums and you'll see your fair share of horror stories of people trying to get a refund or painful customer (no) service tales. Great products, crap customer service - far better off dealing with a stores customer service unless you're forced to deal with DJI direct.

Also, if you're impatient and have extra money burning a hole in your pocket, you can always try eBay. Stock is starting to flow a little more freely now so as more units become available, the premium on in stock ready to ship units go down, so the scalpers can't charge quite as much anymore. You should still expect to pay extra if you go the NOW route for one on eBay, but $1300-$1500 isn't out of the question for a Fly Less package. Generally the longer you wait, the cheaper it will get... although Christmas shopping may bump that back up.