What is an formula to use for half an oval?

Half Ellipse: (x/a)² + (y/b)² = 1{y<=0}

or y=-b*sqrt(1 - (x/a)²)

Graph: https://www.desmos.com/calculator/8axxgnjw5x

For an ellipse with center at (h, k), having the major semiaxis of 'a' units and the minor semiaxis of 'b' units, the ellipse equation is:

(x - h)²/a² + (y - k)²/b² = 1

You mean ellipse?

(x–k)²/a² + (y–h)²/b² = 1 ............. If the major axis is parallel to x-axis

(x–k)²/b² + (y–h)²/a² = 1 ............. If the major axis is parallel to y-axis

Where:

a = length of semi-major axis

b = length of semi minor axis, a>b

k, h = x,y coordinates of center