Gosh, I wrote this tutorial
SO long ago - I keep meaning to write an expanded, updated version...but life just keeps getting in the way!!
The first thing I want to say is there's no right or wrong - trying something slightly different is how you create new styles!! When I do these I usually have an opaque dot covered by a transparent dot. I often use white, but sometimes layering a transparent color over another color will give you a cool color gradient. And I've also done smaller layered dots, just the way Sonja did, for a nice multicolor effect.
Aimee