Here is Jim Brick's take on the zone system. (Don't shoot the messenger)

I contacted Jim and a few others who have published "practical" approaches to the zone system. (the others were Gem Singer and Rob Gray)

Jim said, in response to my request, "Yes, it is I who dreamed this up and wrote it. Not that it was that difficult - it's just common sense. I teach workshops and people were more interested in taking photographs rather than calibrating a film and developer so that they would have control over the process. So I put this together as a quick way to get in the ballpark without having to endlessly photograph gray cards and read the resulting film with a densitometer. And for the most part, it works just fine."

His interpretation of the zone system is quite irreverent, but right on the money in terms of adapting your process to meet your goals.

There are four zones.
Zone Good, Zone Bad, Zone Ugly, Zone Butt Ugly.

To use the system:
Wake up. Get out of bed. Go outside.

Zone Good
It is light overcast, light shadows but good light direction. Normal contrast.
Expose normal (eg: ASA-100 @ 100) develop normal.

Zone Bad
It is dismally overcast, no shadows, perhaps even drizzle. Low contrast.
Underexpose one stop (eg: ASA-100 @ 200) overdevelop 20%

Zone Ugly
The sun is out, sky is clear with puffy clouds, and there are blatant shadows. High contrast.
Overexpose one stop (eg: ASA-100 @ 50) underdevelop 20%

Zone Butt Ugly
The sun is squinty bright, cloudless sky, and the shadows really deep. Very high contrast.
Go in, and go back to bed!. But, if you are a die-hard...
Overexpose two stops (eg: ASA-100 @ 25) underdevelop 30%