First you put the values that describe your telescope in the boxes of the left pane. Then you click on the "Calculate" button and you get a list of off-axis radius, the ilumination for this circle and the light loss in magnitudes.

If the illumination fall below 70% that line is showed in red, so you know than something has gone very bad.

In the bottom of this pane we put the recomended diagonal offset and other info about visual usage.

Then you can then modify the parameters and calculate again the light loss.