Source: http://cookpolitical.com/analysis/national/national-politics/how-define-wave

In 2006, we [The Cook Rpt] had 38 Republican-held seats in Toss-Up. Democrats won 21 — or 55 percent of them.

More important, Democrats won half of those seats (11) with under 52 percent and all but two, with under 55 percent of the vote.

In 2010, Republicans won 65 percent of the Democratic seats in Toss-Up (32 of 49). Republicans won two-thirds of those races by 52 percent or less.

This year, Republicans have 23 seats in toss-up and six in lean Democrat or worse. That means Democrats would need to hold all of their own seats in toss-up (2), win the six leaning their way, and win 17 — or 73 percent of the toss-up seats.

[Italics mine, if you can even see them with whatever theme I’ve picked.]

Most important races are within 5 points, so, like, 47-52 or 46-51.