My reason for guessing that it is a White Rock is that I got her when I ordered a mix of brown egg layers from MPC and White Rocks are the only white chickens that lay brown eggs that I know of.
I guess we'll find out for sure when she lays her first egg!
Her temperament does seem a bit leghorn-ish--she started off very skittish and flighty. Then, after we lost the other chicks that came in her order, she decided that people were alright and became a lovey-dovey arm and shoulder bird but still didn't like being picked up. She would fly across the room to land on our heads and shoulders and it was next to impossible to keep her off, but she hated it if we tried to pick her up and put her there.
Now that she's got her little chickie friends she's not quite so clingy anymore. She also seems to be mothering the little ones--they follow her around and sleep underneath her wings.
I don't think that white part is her earlobe. I think the earlobe is directly under that. You will not be able to tell until she matures a little more.