Yes! I think I've finally worked it out ... 10/25 grind, fill the portafilter until the mound of coffee is just above the lip, then smooth it off and tamp with firm (but not heavy) pressure to pack it in.
Run the water for 30 seconds - liquer starts flowing in 5 seconds.
At the end of it, I have a ristretto sized quantity of coffee that tastes superb! All of the caramel-citrus flavours that I enjoy in a good cup.
I'd welcome any tips from baristas out there who are reading this post.