If you are looking for the "golden ratio", it would be 41.25"+/-. A "chair rail" should be at the height that it protects the wall if a chair is slid back into it. Usually, that is however high the back of the chair is, which mine are around 40". Other visual interests can also be considered, such as if it may tie into a counter top or window sill at an odd height to make it look like a mistake. So, do you match the rail to the room height and buy chairs to match, or match the rail to your chairs?

Agree with Fred, but get the chairs first.