lock spline controls on reference line

[g_vandeel]

I need to create a family for multiple cilindrical storage tanks, so i need to make it a instance family. 
Every top (and bottom) has a spline curved rounding, but i can't lock the spline-control to a reference line. So when changing parameter C, the spline won't follow along. I even tried to lock the "Toggle join tangency"

Any help is welcome

[RC_RVT]

Add a dimension line when you are editing the spline as shown, that goes from the reference plane that the bottom of the "C" dimension is constrained to, to the corner of the spline. When you are placing the dimension line onto the corner of the spline, press tab until the corner node is highlighted. Once this is placed, you can then change that dimension to "C". This should pull the top of the spline down when adjusting. If this doesn't make sense, let me know.

[azad.Nanva]

why didn't you use the arcs?is not possible?

[g_vandeel]

Ah, I understand your proposed solution (wich makes sense), however the dim lines doesnt snap to the control point, Only to the intersection of reference lines. 

Did you test it yourself?

[g_vandeel · Accepted solution]

Arcs doesnt work because I need a smooth transition. 

but your reply made me think! What about a Ellipse and then trim to the edges. this works.