Dear Paul,
The Zone System is simultaneously a simplification and a complication of basic sensitometry.
The naming of zones is a work of genius, as it enables anyone to get a quick, easy idea of what happens on the various levels of the D/log E curve. This is a wonderful simplification.
The rest -- the N+, N-, placing, falling, etc. -- is for my money a jargon-ridden complication.
Maybe three decades ago, I wasted a lot of time on the Zone System because I didn't know enough basic sensitometry to make sense of it.
Maybe a decade ago, when I knew a lot more basic sensitometry (dating back to Hurter and Driffield in 1890), I went back to the Zone System (1940s) and found it to be what I describe above: a jargon-ridden complication.
If it works for you, great: don't let anyone talk you out of it.
If it doesn't work for you, dismiss it without a backward glance, except perhaps at Zone III, the darkest zone with texture (in the original 9-zone system). This represents, near enough, the ISO speed point, and is all you need to get adequate shadow detail.
Meter the darkest shadow where you want detail and give 2-3 stops less exposure: find out by experiment which gives you texture in the shadows. Forget mid-tones and grey cards as no scientific speed system has ever been based on either.
For further refinement, give more development for low-contrast subjects (I suggest 50 per cent) and less for high contrast subjects (I suggest 15 per cent). By all means modify this 15/50 rule however you like, even unto 50/100 or 25/200.
For still further refinement, do full Zone System testing. See if it improves your pictures. My own estimation is that for every one person whose pictures get better, there is at least one other whose pictures get worse because they waste all their time 'testing' and don't take real pictures.
There's a lot more about this in the free Photo School module at
www.rogerandfrances.com about why we (Roger and Frances) don't believe in the Zone System. If you do believe in it, don't let us stop you, but remember that there are at least as many great photographers who don't use the Zone System as do. We do not claim to be photographers in the class of Ansel Adams but equally we are fairly well informed about basic sensitometry. And remember that many of AA's best pictures antedate his promulgation (and, I suspect, formulation) of the Zone System.
Cheers,
Roger