Suppose you have 2 tangent arcs where the end points of the 'combined arc' are 2 defined points on a 2D sketch, the arcs are also tangent to straight lines:

straight line (defined end points) - tangent - arc (undefined)  - tangent - arc (undefined) - tangent - straight line (defined end points)

Now the arcs are only partially constrained. With one more constraint, be it a relation or a dimension, the sketch will be fully defined. However, my aim is to set the length of the first arc to be double the length of the second arc:

straight line (defined end points) - tangent - arc (length X)  - tangent - arc (length X/2=Y) - tangent - parallel straight line (defined end points)

I tried my way by using dimensions and equations, but it doesn't seem to work. If I assign a value to dimension1, then dimension2 is automatically defined (provided the endpoints are fixed), which would break the Y=X/2 link (unless the correct calculated value is known). And I can't leave a dimension out, because then I wouldn't be able to select one of the lengths to create the equation. If I delete a dimension, the equation is broken because the dimension doesn't exist

Can a dimension be created without value? Or is there a way to link both dimensions to each other without assigning a predefined or entered value?