I am making an atlas with a fixed scale, and want to include circles with a fixed radius (in scaled kilometers) about the center point in the atlas (see image below). I was able to do this by calculating the appropriate scale using the scale bar and a physical ruler. However, it seems like there should be a more precise way to solve this problem.
Within the expression builder of the circle height and width, the scale of the map can be retrieved with:
This returns the scale (30,000), however when I try to scale the target distance (2000 m) by this scale it doesn't match length of the scale bar (also 2 km) -- instead it is about 2/3s of the target radius.