Nat. Methods 12, 131–133 (2015); published online 15 December 2014; corrected after print 6 February 2015

In the version of this article initially published, the Hendrickson formula in the Figure 2 legend incorrectly had (2NA/NP)1/2 divided by (f''/Zeff); these terms should have been multiplied. The error has been corrected in the HTML and PDF versions of the article.