Erich, TriX is correct in that each camera is different, even the same model camera may have slightly different capabilities. Controlled testing is the only way to determine where your camera gives up capturing details on the high end. Personally, I wish it were easy to change from one Zone to another by simple shutter speed or aperture change. Unfortunately, it's not that simple.

Testing is the method to determine where that failure point is. At that point, one can determine the number of Zones per stop. One of my cameras takes just over 2-1/2 stops to reach a highlight boundary that is unretrievable. This means that 5 Zones are covered by 2-1/2 stops. Thus, if I want to use specular reflections in the scene for metering, I can increase the exposure by just over 2-1/2 stops. If I spot meter on the brightest part of the scene with visible texture, I'll increase my exposure suitably to place that part of the scene in Zone VIII.

In processing, I'll use the histogram to make the exposure adjustment to compensate for the additional exposure during the initial capture. But, without testing it's a crap shoot as to how much additional exposure to give an image at capture time. I use this method 99% of the time when doing digital photography.
--Bob