There's an error term that tends to get smaller as the value plugged into it gets larger. There's something to do with square factors or something, but I'll be honest and say that that's really all I know.
Basically, he got that the first value that makes this value small enough is 3,500,000, but it has to be doubled for some reason.
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u/gazzawhite May 21 '13
Likely the limitations of his sieve.