Free Float of Activity Formulae ?

What is the actual formulae for calculating Free float.I have both of these formulaes used at different places. 

Should it be
1. ES of Following activity - EF of Present 
2. ES of Following Activity - ES of Present Activity - Duration of Present Activity

Using either of the forumales results in different answers. 

Please refer to http://www.deepfriedbrainproject.com/2008/12/pmp-certification-exam-prep...

The concept is explained in this page

Have already gone through that but my point is how many agree with him because there are multiple options all over the web and PMBOK doesn't clarifies this.,