Yea! I'm glad you decided to go for it. That's a super cute pattern. (I thought a waistcoat would be a vest but I wasn't sure.) Ingrid's right, it will be a great opportunity. I think the 6-12 mo size would be the better option because babies grow so fast. It would be good in November IMO but as you'll have it done sooner it will probably be worn sooner. Babies need to be kept warm but I learned early on that if I kept mine too warm they got sick.
I should never overestimate my basic arithmetic skills.