I thought it is too obvious to say using radius of the sphere xD
We should use it of course, I mentioned the radius of the area cause I need it to be involved into the formula
That picture is the best image I found on net that suits my question so I put it there, otherwise I don't need physical or chemical information (question is not related to these areas)
And I found what I've been looking for (curious ones can Google the term "spherical cap") so please inform me if I need to do sth to close that page
Thanks for trying