Follow the numbers on the left side of the dish, standing behind it. Declination is what you subtract from 40 (this is the motor bend for this motor, if it is indeed an SG-9120), and you use that value to set the dish elevation. But this value MAY be "flexible". In other words, you may start getting signal on an elevation that doesn't really match what your elevation should be in the book.
So, the motor is set for my latitude on the motor bracket (a straightforward adjustment) and then the dish is set to 40 minus the declination using the settings on the dish bracket (and the numbers on the dish bracket are multiplied by 10 - obviously a coarse adjustment)? Where can I place an inclinometer to verify settings?