This may help...I have never set anything to .31 as I don't know how.
Footprints by Dish Size - Adjusting the Polar Mount for Prime Focus Antenna - C/Ku-Band Satellite Systems - Tuning, Tracking, Azimuth, Elevation, Declination Angles, F/D Ratio, Focal Distance, Inclinometer, LNB/Feedhorn Assembly, Actuator Assembly, C
The DMX741 has markings on the side, but not the best. If you have the original feedhorn, you can use that as a guide.
Also, are you aligning the DMX's "zero" mark to the Dishes Axis?