There is some interesting math here.
Begin with X and Y coordinate axes, and draw the square with coordinates (0,0), (0,1), (1,1), and (1,0). Now inscribe a circle into the square, which will have diameter 1 and radius 1/2. Place the point P in the middle of the circle, with coordinates (1/2, 1/2).
Now draw a line bisecting the circle, from (0,1) to (1,0), which will be two corners of the square. Now pick a point to the left of P, for example x=1/4, and draw a vertical line through it. This line will intersect the bisector at coordinates (x, 1-x), in this case (1/4, 3/4). Call the point of intersection R, and call the two points of intersection at the bottom and top of the square A and B respectively, for short.
Now look at where this line intersects the circle. These coordinates will be (x, y1) and (x, y2), where y1 and y2 are on the bottom and top of the circle respectively. Call these points C and D for short. Now we can start doing math. Recalling that x is where our line is, we can use R as a fulcrum and ask "what weight on each side of the fulcrum would balance a scale". Done this way, we have len(A,R) = 1-x and len(B,R) = x, and now we can slide x anywhere between 0 and 1 and these relations will hold. Therefore we have
(1-x)/x = (y-y1)/(y2-y)
Thus
y = xy1 + (1-x)y2
And therefore C and D satisfy
(x-1/2)^2 + (y-1/2)^2 = 1/4
and rearranging we have
y1 = 1/2 - sqrt(1/4 - (x - 1/2)^2) and
y2 = 1/2 + sqrt(1/4 - (x - 1/2)^2)
Now substituting into y we get
y = 1/2 + (1-2x)sqrt(1/4-(x-1/2)^2)
which is the equation of the S curve separating the two halves of the yin and yang.
This curve represents "perfect balance at all points". No matter where along the curve you are, the weights on either side of the fulcrum are balanced.
We could take this further to figure out where the fish eyes are. The min of S is 1/4 and the max is 3/4, so if the eyes are horizontal like in the OP their coordinates are
1/2 - sqrt(2)/4, 1/2 and
1/2 + sqrt(2)/4, 1/2
You can verify all this with two buckets of water and a stick, and I'll bet that's how the ancient Chinese did it. They didn't need square roots to figure out the right points, just water and sticks.