Limit of star magnitude.
If you want to obtain best results, you should use reference stars not more dimly than limiting star magnitude of your telescope. (Limiting star magnitude depends on time of an exposition also.) This value should be set as "maximum of star magnitude" in a dialog box "Extraction from catalogue".
To find darkest stars in the frame, make the following:
1) Open a file.
2) Identify a field. Thus choose catalogue USNO. This catalogue contains darkest stars.
3) Press "i" on the toolbar. The window "Information" will appear.
4) Choose a darkest stars. Click it. Two star magnitudes (m1, m2 in two photometric systems) will be outputted in the window "Information". These magnitudes will be close to limiting.