Building the Foundation of the Design - Autodesk Inventor 2010: No Experience Required - page 71

4. Place a dimension between the right endpoint of the bottom line and
the end lowest point of the line that you just made vertical. Edit its
value to .125in, as shown in Figure 2.19.
FIGURE 2.18 Converting a parametric dimension to a driven dimension
makes it associative.
FIGURE 2.19 Adding the dimension for the gap fully constrains the sketch.