Congrats on the purchase.
The grind you will need does indeed depend on a few factors and will change as the days (and even the hours) pass.
Starting point - Most grinders markings will differ, so you will hear talk of how the grind feels in your fingers. To begin, try to get a grind that feels like beach sand. Do your stuff and see how it comes out. To begin wth, try for the 60ml in 30 seconds (for a double) from the time you hit the brew switch on the Silvia.
If it chokes the machine, grind a bit coarser and if its a gusher, go a bit finer.
You do get the hang of your own grinder pretty fast and you get to the point when you know if a big or small adjustment is required.
Whether you decide to sink a shot or make the adjustment for next shot is up to you and largely depends on how far out it was.
As beans get older and weather conditions differ, the grind usually needs fine tuning. Again, you will probably pick these things up the more experience you get with your setup. Often I will make slight adjustments when using the same bean over a week, but when I move to a different bean, the change can be more dramatic.
There really is no magic setting. For great coffee at home, its a continuing process of monitoring, learning and above all enjoying!