Setting markers of calibration is hampered by saved (ignore) zones been rendered opaque which make it difficult to set markers separated far enough apart.
Maybe used a different render method for unsaved zones to keep to distinction.
A workaround is to define the calibration markers in a new profile. Then copy the markers from that profile to the active one. Of course, it means you can't visually check whether all the markers are still aligned
This will require reworking of the displays as the overview is masked by OpenCL before being passed back to the UI