At The Present Time I Take 200mg Of Carbamazepine Twice

At the present time I take 200mg of carbamazepine twice a day. Because of my insurance I now have to pay for this medication. My doctor is an interist and is not that familiar with seizure medication. After checking with what Humana will not charge me for, I find that Phenytoin is the one best for me, I have taken Dilantin in years past. My question is, what dosage should I take that would compare with 200mg of Carbamazepine at twice a day?