Two Thirds

The biggest difference between focalized and generalized seizures is that if Damonito had focalized seizures they could remove the part of the brain with the seizure activity and his seizures would stop. With generalized or "whole brain" seizures if they can stop the misfire from one half of his brain to the other there is a chance that the seizures will also stop.
The problem with the surgery is that if they cut the entire corpus callosum then the necessary communication between the two halves of the brain is also cut. Which means things we take for granted, like reading, would be impossible. So they cut only two thirds of the corpus callosum with the hope that is enough to stop the misfire. The theory is if two thirds works then seizures are controlled and the child could have some hope of normalcy. If the seizures continue, then consideration must be given to cutting the entire corpus collosum. Since Damonito's surgery only reduced the number of seizures without eliminating them, we have a lot to consider.