Prong setting for an oval stone

I searched the forum and while I could find other information that will help me, I could not find anything particular to this subject.

The problem I am trying to solve now is how to fit 4 prongs for an oval stone. The problem is not really about how to fit them technically but more about the simmetry. How do you get this done? Do you do it by eye?

I am talking about a ring like the one in the picture.