Using Splines in Solidworks
Firstly, don't use too many points, these will need to be controlled and will make your curve less smooth.
This example is not so good...
This can be seen with the curvature comb tool.
Rigth click the curve and select the tool.
Notice the lumps and bumps in the comb (exaggerated).
Add a two point spline instead (don't add points at the middle).
This will look like a straight line to start with, just grab the handles to manipulate the curve.
Add relations and dimensions to the curve or handles.
I have related the curve - tangent to the two centrelines.
I have added a length dimension to the top handle.
You can also add angle dimensions to the handle from another line if wanted.
The two point curve, fully defined is near identical to the original curve but is much smoother when checking with the comb tool.