Drawing an ellipse outside rectangle, what is the math for this?

Drawing an ellipse outside rectangle, what is the math for this?

I drew a rectangle, and now I want to draw an ellipse outside it so that rectangle fits barely inside the ellipse.

I don't want a perfect circle around the rectangle, that I can do with diagonal length.

I want an ellipse, just barely going around the corners of the rectangle.

Here's my code, and edges of the rectangle are stretching outside the ellipse.
How can I fix this?

If the ellipse is slightly larger than the...

Drawing an ellipse outside rectangle, what is the math for this?

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