Let's first get the conventions clarified:(posted 8786 days ago)Zones I to III are the shadows, with zone III giving full shadow detail, zones II and I giving no detail any more.
Mid grey (18%) should be zone V.
Depending on the amount of detail desired, the highlights should be somewhere in zone VII (full detail), VIII (no detail, but not yet paper base white) or IX (usually paper base white).
Meters are calibrated to zone V.
So, when you meter the fully detailed highlights, zone VII, and close two stops, this will get you to zone V, which seems fine so far.
Metering the fully detailed shadows, zone III, and closing two stops will get you there, too.
The third way to get there is measuring the gray card and taking that value as-is.
Now for the catch: If the scene is rather contrasty, the range between what you want to be fully detailed highlights and what you want to be fully detailed shadows may be more than four f-stops. When you come from the highlight end of the scale, this will let the shadows slip into zone II, which means they drown in black, no detail any more. The highlights are zone VII, which is fine.
Otoh, when you come from the shadow end, metering for zone III, and finding that the highlights would have no detail any more, you can modify your development to pull the highlight density down as required w/o losing shadow detail. With some film/developer combinations which do not have a shoulder in the characteristic curve, you might also develop the film as usual and try to get detail into the highlights by burning them at the printing stage.
OK?