Try creating a trimmed ellipse in a sketch (with an axis line),
then "Revolve' the sketch as a revolved Boss or revolved Cut.
You cannot control specific radii in a parabola, because it's
radii are infinately variable. So instead you control the
theoretical overall size of the parabola, by constraining the nodes on
a trimmed ellipse.
Revolved ellipses are also really useful for creating organic-looking,
muscular features on base surfaces:
You first create a knit of the base surface as a trimming tool.
Then revolve an ellipse to create an adjustable organic bulge
intersecting that original solid. Lastly you trim the revolve as needed
using the knit surface that was created before the revolve.