"Chair rails" are usually around 34" - 38" above finished floor to the top. A light switch is usually 42" - 46" above finished floor to the bottom. If you are trying to split the wall in half with a wainscoting, it tends to look fairly plain once you get around the entire room. The more common and interesting ratio is closer to 3/8, so that the rail is either down at 3' for chair rail, or up at 5' for a picture rail, or even higher for a plate rail. There are usually several other elements in a finished room to key off of rather than half the wall height, or how high the switches are. Sometimes, the height can be determined by a window. It usually doesn't look right to have a wainscoting stop just below a window. It should gently cradle the window. Higher than a backsplash is also a bit odd. The furniture that is going to be in the room, or towel bar heights should also be considered.