It should be the height of the top of the back of the chair you want to stop hitting the wall. Thats if its there for a practical reason, if its just decorative then its entirely up to you but the height of the chair is a good start.
Thanks DaveD , it will be more for decoration purposes as we will not be throwing the chairs around and it will act more of a break between the 2 colours in the kitchen however it will also have to be pratical if we sell up.We need to get a new dining room table and chairs so this will be on the back burner for a while .