Most rituals required some form of frame, usually a runic matrix, to set the parameters. The exact parameters and runic clusters could generally be calculated with the help of arithmancy. If you set the parameters wrongly or in a way that did not compute, the outcome would also vary greatly, almost always in a very negative way for the person undergoing the ritual. Rituals were not a precise form of magic, yet a small mistake or deviation could easily have deadly or irreversible consequences. Not to mention that undergoing multiple rituals was incredibly dangerous, and he had to perform them in a certain set of potent magical numbers to avoid instability.