The Zone System is a Visualizaion Tool.
Yes, it helps you get the right exposure, but that's not it's primary purpose. The whole idea of correlating stops of exposure to shades of grey in your final print is so that you can decide before you expose what you want your print to look like.
The trick is getting used to the feeling and the expressivity of the different print Zones so that you can successfully plan. If you place a shadow in Zone III, but really wanted a Zone IV shadow, you've just underexposed a stop. If you develop to place a concrete wall in Zone VIII but really wanted it in Zone VII, you've overdeveloped, and so on.
Practice with different placements, learn what a Zone III shadow looks like in comparison to a Zone IV shadow (I even place some shadows in Zone V on occasion); learn how much detail you get in a Zone VIII, VII, IX, etc. and how bright all those highlights look.
And, learn to use your meter to help you "see" what your print will look like. If you place a shadow in Zone III, for example, and then most of the other values in the scene except for a highlight are in Zones IV and V, you're going to have a pretty dark looking print... etc.
When I was learning the Zone System, I made Zone Rulers, strips of photo paper exposed and printed to all Zones between black and white for all the developments I used (See "The New Zone System Manual" by Minor White et al. for instructions on how to make them). These were a great reference for me until I could carry the shades of grey around in my head.
And, as ROL says, learn to look at the light in your scene; you might be able to expose to get a Zone III shadow and a Zone VIII highlight, but what is the rest of the scene going to look like when printed.
Oh, one more thing: don't get discouraged. You managed to choose two of the hardest lighting situations to deal with in the world. Filtered light through the redwoods is really contrasty, but the feeling is luminous and soft. It requires a bit of virtuosity to deal with this situation successfully. The same goes for the harsh, very contrast sunlit alpine scene you had at Crater Lake. FWIW, I am an Oregonian and I've been trying for 30+ years to get a really expressive print from Crater Lake... I still haven't managed to get one I would exhibit.
Best,
Doremus
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