David,
I have been very busy lately and just saw your response. Thank you for your kind words. Yes I do enjoy clouds in my images, as well as water. I think they add additional dimensions and moods to images.
Their are varying factors that will determine strength of filtration to darken skys; near cities where air pollution is higher, stronger filters are needed to darken the sky. The higher the sun in the sky, the weaker the filter. What is under the sky will determine the filter, rocks, buildings vs trees, foliage, water. In b&w photography, to over filter with too strong a filter can ruin a mood and is the most common mistake.
The best way to learn is by experience and keeping good records of what and how you photography. Exposure, filter factor, type of filter, placement of values on the zone system scale, lens, film and speed, development, etc, will determine the final outcome of the photograph.
I do not know if Zone VI through Calumet still sells exposure record notebooks but they are very handy. Or you can go to Ansel Adam's book on the Negative and there is a form there you can copy.