I have a question related to how to calculate activify free float.

The fist activity in the diagram normally has an early start of zero, but I see some people use 1 has the early start of the first activity. Which one is correct? If I use Zero, then my free float comes one less.

Also, it sounds like if I srart first activity with 1, then I have to use ES = EF of successor + 1 formula.

Can someone please help? PMBOK does not specify anything on how to calculate early start.

Thanks.