You can capture the symmetry of the knob in the design intent of the part. You build one half of the sample part in this tutorial, then in the Fillet Features tutorial, you mirror the model to create the other half. Any changes you make to the original half are reflected in the other half.
When you relate features to the origin and the planes, you need fewer dimensions and construction entities, and you can more easily modify the part.
Click New
on the Standard toolbar
and open a new part.
Click Extruded
Boss/Base 
on
the Features toolbar and open a sketch on the Front
plane.
Click Centerpoint
Arc 
on the
Sketch toolbar.
Drag downward from the origin, then release the pointer.
A circumference guideline appears.
Click and drag an arc approximately 180° counterclockwise around the origin.
Draw a vertical line from the bottom endpoint of the arc, through the origin, and ending near the upper endpoint of the arc.
Press escape to release the tool.
Hold down Ctrl
and select the upper endpoints of both the line and arc.
In the PropertyManager, under Add
Relations, select 
Merge.
Dimension the arc radius to 15mm.
