I agree that seeing the cords is the indicator.
You could find out the thickness of rubber from the manufacturer, measure the new tire, then replace when it is at (new thickness - 1mm). But I think you will find that tires are not manufacured to extremely tight tolerances and/ or they don't wear evenly. When I see cords on mine, it is usually in a small patch or two. Not evenly around the whole tire. So you would have to take measurements all around the tire.
Or, instead of buying a micrometer, taking your tire off once a week, and taking a dozen measurements, you could just look at occaisionally.