The article states that the total deficit for the first 8 months was $12.8 billion. Using your approach and ignoring trends, the annual deficit would be $12.8 + $6.4 billion = $19.2 billion - less than half your figure! However, you can't estimate the annual deficit from either a single month or even 8 months, you have to look at the trends and have to have access to ALL the underlying data that Finance has at its disposal.