Couple more things
As for how thin you want your paint, it should be thin enough to flow smoothly without leaving clear brush marks. You can always test on a sheet of paper or your thumb.
You can thin your paint more if you use a pallet with wells instead of a wet pallet. With a wet pallet, if the paint is too thin, it will soak right thrrough or run everywhere.
For another, load your brush very lightly, so you have more control when you apply the paint. Blot on a paper towel, damp sponge or bit of old cotton t-shirt.