How thin/thick should the water and Acrylic Ground for Pastels mixture be when applied to
cold-pressed watercolor paper?
The paper's absorbency is going to be a factor for smoothness of the Acrylic Ground for Pastels. You can either seal the surface with Gesso, or better yet with a medium, such as Matte Medium, Polymer Medium Gloss, or a thinned Soft Gel.

Also, the actual dilution can be changed to better suit your needs. We usually suggest thinning by adding 20% water, but if you experience leveling problems, add a little more water to help the flow.

You may also want to try some various brushes. The gritty quality of this ground adds some different brushing qualities. Sometimes a stiffer brush is better for the Acrylic Ground for Pastel instead of a softer one.