A precinct is an individual voting location. The ones I've worked at usually had less than 1200 total votes. That's 8% fraudulent. That's significant, and it is wide-spread enough to be found in at least those numbers in 17 places in the closest race in the nation. It's a story. Let's hope it has legs.
Your math is correct, however your assumption that the only fraudulent votes are those in excess of the number of registered voters is not.
80% turnout would normally be considered "very high" in any precinct. In a precinct of 1200 registered voters, 1300 votes is a turnout of 108%. The fraud is most likely closer to 30 or 40%.