As a guide, a bar sits 50mm to 100mm above the top edge of the mirror, which on a standard vanity lands it around 1,900mm to 2,000mm from the floor. Sconces mounted beside the mirror are centred at eye height, roughly 1,500mm to 1,700mm, and a tall household goes a little higher. Your electrician sets the final position, so give them these numbers with the fitting.