The brief answer is, as accurately as you'd like the DSCs to operate. In addition, the optical axis of the tube must be perpendicular to the elevation axis. Again, the short answer is, as accurately as you'd like the DSCs to operate. One can't do the job with a hand-held drill. On the other hand, careful work with a modest lathe and drill press is quite sufficient, especially if performed by a modest machinist.
Most astronomy clubs have such a person.
Best accuracy is obtained with high-resolution encoders. Standard encoders have 2048 steps per revolution, and high-res type has 4000. One can also use gears to provide greater resolution, but see below. If the encoder is connected directly to a shaft, the hole in the shaft must not be oversize. It must be straight, well centered, and parallel to the axis.