The amount of bending in your bracket (is it steel or aluminum?) to take up the out of centerness translates directly into pointing error in your encoder. If you have no bending, you can hope to get 1/10 degree accuracy with typical digital setting circles. If your bracket bends a half of a degree, you can now only hope for one half degree of pointing accuracy.
Unless you have a well equiped machine shop, it is probably a good idea to buy a bolts with precisely drilled holes. All the digital setting circle distributers sell them in their mounting kits.
Chuck