It's a beautiful dress! Congrats on starting a new weight loss program. I was told Maggie dresses take 12 weeks to come in. Check with your dress shop on when you would need to order to have in time for your wedding.
I would wait a while before ordering, to see how much wait you can lose. That way there will be less to take in later.
Basically, I would not order it too small. A dress can always be taken in, but very rarely can it be let out. If anything happens, and you don't lose all the weight you wanted to, you may not be able to wear your dream dress!