It is going to be the either the "top"pair of holes that is straight across from each other, or the "bottom" pair.
If you already have your pole set go ahead and install the mount without the reflector and drive it to apex, set your elevation, and then measure your declination with the bolts set in one of the pair of holes I described above. If you don't have your pole set you can make the same measurements if you can get the mount to stand up on any level surface.
You may have to fine tune your elevation slightly once the reflector is installed, but once your declination is set it is done, you will never change that.