Taking the US population as 300 million (to make the math easy), that'd mean that 150 Americans know that statistic, and if one in six of them dies (I'm assuming that Munroe means rate of death by lightning strike even though he just says death rate) of lightning strike in a year, that's 25/year who know the statistic and die.

Taking the cartoon's stated figure of 45 Americans killed by lightning each year, which should include the 25 described above, that means that 55% of lightning deaths are people who know the statistic.