if the ring has to be "perfect" a few extra steps will go a long ways. On the ID cut the Id smaller than the target by about .01-02 of an inch to remove the slug. Once you have the slug removed have the program burn at normal offsets for the target Diameter. With these passes (yes the more the better) arc onto the part and arc off the part. at the end of each pass rotate the program 45 Degree so that each pass will travel over the previous witness mark (ie 0 degree,45 degree,90 degree,135 degree 180 degree ect). I am not familiar with charmillies power settings but with a Sodick i rough with larger offset and then use a Water5 tech (high energy rough with 3 skims and one Pika pass) and have had several customer parts come out .00005 or better when checked with a CMM. Depending on the material you may not have to do the first pass that removes the slug with additional offset and the number of passes may also be less. Are you also trying to make a "perfect circle on the OD?