Egg decoration

A good equation of an egg is:

$(x^2 + y^2)^2 = a x^3 + (a - b) x y^2$ where $a \ge b \ge 0$ and $b=0.7a$ .

Our egg has $a=2, b=1.4$

In that equation, we can isolate $y$ and differentiate the function. It will produce value of $x$ maximum (radius of our circle) $x = (27 \sqrt{561} + 573) / 1120$ The exact value is of the circle circumference is: $\dfrac{1}{560} \pi \sqrt{6 (2431 \sqrt{561} + 52089))} \approx \boxed{4.55}$