For unscored pills that don't need to be distributed evenly (although my experiences may contradict this.... for HR purposes I will not say that they distribute the morphine evenly because that's a serious HR liability, my experiences are subjective and are my own only. Others may react differently, and should remember that I am not qualified to make statements like that, I don't feel comfortable doing so because I don't know how the pharmaceutical companies are making the pills), like MSContin which by design shouldn't be split, crushed, etc because it affects the time release in a dangerous way.
In my experience though, I'd use a pill cutter to divide MS100ER into 4 pieces of 25mg and then weigh each piece on my milligram scale. Why do you say 18mg? I never had any problems with the 25mg dosage ha.