First of all, B vitamin or folic acid is a great thing to give your dog, when she's expecting. [Ye]
A normal dose would be 400 mcg for a large dog, 200 mcg for a medium dog, and 50 mcg for a small dog.
But please not, this is based in it's starting wieght, and not including the weight it's put on during it's pregnancy. No matter what, it's sounds like you have a rather large dog, and you could easily give it somewhere between 300 and 400 mcg.
Best of luck to you, your dog, and your future puppies.