How do you use a doctor-style weight scale?

The scale at my gym is one of those standing numbers, with a 50 pound weight, and a 1 pound weight, that you slide, and the thing tilts within a cutout square. So, where do I want the thing to be!? Touching the top, bottom, or right in the middle? And how can I tell if it's in the middle without doing some sort of ridiculous limbo move, since I'm over 6 feet tall? It's not exactly at eye level.