As stated in the opening paragraph, a meganuclease with an 18-base pair sequence would on average require a genome twenty times the size of the human genome to be found once by chance; the calculation is 418/3x109 = 22.9.
For example, a sequence that is identical in all but one base pair would occur by chance once every 417/18x3x109 = 0.32 human genome equivalents on average, or three times per human genome.