The three digits in chmod refer to the USER GROUP AND OTHER permissions respectively. Depending on which process is running your webserver, you will find either 755 or 750 work. If the webserver is in the group that owns the public_html directory then 750 will work (since 5 is referring to the group having read and execute permissions). Otherwise you will need 755, so that others (including the webserver) can access it.
Hope that helps you all.