Is there a way to have a sketch dimension linked to another (or a variable) in one configuration and be just a straight value in another?

Here’s the backstory…  I have a series of axes that are defined by the Top Plane and a series of points.  The points are defined by the intersection of a curve and lines (in another sketch) so that as I change the radius of the arc the points will move along the lines.  This works fine until I want the points to be in a straight line because for some reason I cannot make my arc have an infinite radius.  My hope was that I could have a second line 12” above the straight line I want and use it to dimension the points to.  That way, if it is in a curved configuration I can make the points linked to the reference dimension of the line plus 12 and in the straight configuration I could make all of them 12.