Upload & Sell: Off
Metering off of a sidewalk, grass, etc. sounds like a suggestion someone made for situations when for some reason you can't meter directly on the most important area of the scene (e.g. you're too far away to confine the reading to that spot). It has nothing to do with the zone system. With the zone system, film, and a one degree spot meter the usual method of metering to obtain the desired exposure was to meter on the darkest important shadow area in the scene and then reduce the exposure by one, two, or three stops depending on whether you wanted to place that area on Zone IV (dark gray), Zone III (darker, almost black, gray) or Zone II (black).
But metering with a digital camera is entirely different. With film the concern was to make sure you didn't underexpose and lose important shadow detail. Which is why the exposure was based on the shadows. With digital the concern is the opposite, i.e. to make sure you don't overexpose and blow out important highlights.
While I used a one degree spot meter and the zone system of exposure and development with film for many years, I've never found it necessary or particularly useful to use either one with a digital camera considering that the histogram tells you everything you need to know. But if you want to use one, just remember that the aperture and shutter speeds given by the meter will place the metered area on roughly the equivalent of a middle gray. If you want the area to appear darker than middle gray reduce the exposure by one (darker gray), two (much darker, almost black, gray), or three (black) stops. If you want the area to appear brighter increase the exposure by one (light gray), two (normal white), or three (bright white) stops.
I never found gray cards particularly useful in the field, mainly because you'll get very different readings depending on the angle at which the card is held to the light source. The same is true of your hand.