I am not a construction expert, but those values aren't an innate characteristic of your soil. The moisture value is just a "snapshot" of your current moisture conditions.

However, those figures DO actually sound about right for optimal compaction. (Generally, the higher the clay content, the higher the moisture content required for optimal compaction.)

You can test it yourself. Grab a sample of your material. Can you make a cohesive ball? If so, then good moisture content.

Can you roll out a pencil of material that does not break up? Then good moisture and good clay content.

When you actually start construction, the moisture content is going to change! Once you dig out your subsoil material and start moving it to the dam, it will begin to dry out. As soon as you get a rain that soaks your core trench, it will be too wet.

You will have to adapt to conditions during construction. However, it is almost always easier to work when it is dry and then add water as needed. Working during a rainy period on wet clay will cost you lots of time and money. (And may make it impossible to compact the core trench.)

Hope that helps a little.