Really depends on the size of your head, we all have to do the twist and drop method when putting the helmet on. I like having more return edge than less, people can't see as much under the helmet, especially younglings.
I normally trim so it's curved on the sides so no sharp areas,/corners. Loosely add your S trim and see if your noggin (head) will still go in, if not trim a little more.
Ear gap references