Start from the ring size
The available circumference is set first of all by the size. Only then can you check how the stone size and setting type fit into it.
And when the layout doesn’t close?
The goal is not to accept a gap and hide it. Look at the relationship between the stones, the spacing and the height at which the stones sit relative to the finger line. Changing that height also changes the radius and circumference they sit on.
Don’t solve the math at the expense of the piece
You can’t simply raise the structure to gain circumference if the result is too tall or uncomfortable. The solution has to stay right both visually and physically.
Why is a full eternity sensitive to resizing?
It usually has no free area that can be lengthened or shortened without affecting the stones. A new size may require a new stone layout or a re-fitting of the structure.