In the sketch, you can set an odd number of control points of this spline (e.g. 15) and select the midpoint to constrain your plane and if you want to set it normal to that spline.
Checkpoints --> right-click on the spline --> simplify the spline --> decrease or increase the tolerance to have an odd number of points
Above the 2 captures. The 1st is the front view and the second is the bottom view (Z axis)
I need to measure values on the front view. However, since the spline also varies on the Z axis, I would like a plane that would be the average of all the variations on this axis. In order to have measures that are as favourable as possible.
I'm not sure I understand, but you can do constrained planes at each end of the curve and parallel to the front plane, and then do a middle plane, and do the same thing again but parallel to the plane below, and the intersection of the two middle planes should give you an X axis in the middle of the variation of your curve between the plane below and the plane in front.
It's twisted, but I understand your request like that, I just?
On the other hand, it has nothing to do with the center of your curve...
The method I gave you earlier can allow you to create a constrained plane according to a point in the middle of your curve, you can from this point define the orientation of the plane according to the desired view.