Lehr's Equation (Rule of 16) is generally a good estimator. It is explained (but not referenced) in a similar article here: http://www.evanmiller.org/how-not-to

Lehr's Equation (Rule of 16) is generally a good estimator. It is explained (but not referenced) in a similar article here: http://www.evanmiller.org/how-not-to-run-an-ab-test.html http://www.evanmiller.org/how-not-to-run-an-ab-test.html