Spaceclaim, how to set the axes of an ellipse

 

Hello, I have to build a nozzle with an elliptical output section in 3D whose dimensions of the a and b axes of the ellipse are variable (but the area does not vary). So I want to set a and b. 

When I use the pull on my ellipse option, I change a and b at the same time. When I use move, I can't just vary an axis without breaking my ellipse. Can you guide me if you master the software well? 

Thank you in advance.