If the bull is good and the heifers are in good shape and cycling there is no reason that you shouldn't get 95+%. However, a bull likely won't get them all bred in one cycle. With AI the success rate can be extremely high or extremely low. It depends on a number of factors including how well one heat detects, proper technique, proper placement, etc. There is no reason that a good experienced AIer shouldn't get 75% on the first cycle. Realize that it takes more labor to AI than to run a bull with the heifers. Many people will agree that the added genetic improvement from AI is worth the extra time and effort. I personally would AI heifers the first cycle or even two and then use a cleanup bull.