Constructing section curves for a surface using multiple points and splines in a 3d sketch as image

I can set end conditions on the splines which are connected to the edges and control the bulge with the sketched point.

If I try and set tangent end conditions for the curve adjacent to the surface rather than edge then the point repositioned by the system - without end conditions the point can be any where in 3D, set end conditions and it moves th point.

Any ideas?

can't set end conditions on a 'curve thru' ref points'?

Am i going to have to build a plane and a 2D curve?

Cheers, Sean

sw2013