Well, I base that on 2008 total turnout of about 5.6m votes. Obviously more or less could turn out, but I think the recount in OH is 1/2 of 1% (about 25,000 votes with the numbers we're using).
In 2008 there were 200,000 provisional ballots cast; and it is estimated that military and overseas ballots number about 20,000 this year.