Setting 1 on my Lux (ie 3rd click from zero) with a decent tamp seems to work best for my Silvia when using Genovese or Atomica blend. Occasionally it will choke at this grind if I tamp really hard or perhaps if the beans are extra fresh, or perhaps because of temperature/humidity? But usually gives me 50-60 ml in about 25-30 seconds with a 14gr double basket.
I notice that with home roasted beans I need to grind a bit coarser for the first day or so, but never coarser than 2.1.
If I use the stock-standard Rancilio double basket (but keep the grind/tamp the same) the coffee comes gushing through a pale yellow and takes about 10 seconds for 50ml!!
Maybe my Lux grinder is a freak (Its officially a Nemox brand - could this account for the difference?). Maybe my Silvia is a freak.
It would be interesting to hear from other Lux/Nemox + Silvia users.