Indeed, great shot. The apertures that should be used for your purposes follow the usual optical laws of photography in terms of subject-to-camera and depth of field (DoF). The closer the subject is to the camera, the less the depth of field. With a macro lens, this is even more prominent due to how close the focus can get with the lens (in the order of 31cm on the FA 100/2.8 macro). This close focusing makes what you'd expect to be a good aperture (like f/8) unusable due to the paper thin DoF. Diffraction is an issue for most lenses above f/11, but this lens shines at least to f/22, and I have barely seen any degradation in image quality even at f/32.
An example of how thin the DoF can get at close focus:
This image was taken at 35cm or so at f/32 - note how thin the DoF is in this image.
The trick is to have as much of your subject as possible to be on a perpendicular plane across the photographic field, to ensure DoF is sufficient at these critical focusing distances to capture all of your intended subject in focus.
Other examples of what this lens can do: