As the name implies,exactly the height neccessary to stop your straight back chairs from ruining your wall covering's.Not the precise answer you were looking for,but the principle still applies.As a functional fitting I'd place it where it would most protect the wall coverings,ie furniture height.Unless you have dining room chairs and require original use the choice is yours.Most if not all wood features intended use was to protect from accidental dammage,ie skirtings were/are to protect delicate plaster from brooms.If none of this is relevant I'd place them where it looks right,say 36".
caine, August 2008