I don't know why but this forum isn't allowing me to paste. You can dimension the length of a spline to retain your 10mm length. Then add points, or use existing points, to set your bend or shape.
Attached is a spline inside a rectangle.
The spline end points are horizontal to each other and are symmetric about the origin.
The spline end tangents are dimmed and one is set equal to the other by an equation.
The spline midpoint is vertical to the origin.
The rectangle is related to the spline end and mid points.
The spline length is dimmed and I think you'll find all updates accordingly as you modify dims.
I hope this helps.