To Dimension a Conic Arc Using rho
1. Click Sketch > Normal or right-click and choose Dimension from the shortcut menu.
2. Select the conic arc and then middle-click to place the dimension. The default rho value is 0.5.
3. You can modify rho as follows:
For a parabola, use the default value of 0.5.
For an ellipse, type a value between 0.05 and 0.5.
For a hyperbola, type a value between 0.5 and 0.95.
* 
When you create a closed ellipse section from four conic segments, type a rho value of (sqrt (2)–1). Do not use the approximation 0.4142 for the square root value.