Welcome to Coffee Snobs.
No two EM0480s are guaranteed to grind to the same level of fineness if set to the same number so I cant answer that question.
Especially since I dont have a Silvia.
The instruction book for the 0480 suggest around 13 IIRC.
When you say 10 shots all too fast or too slow, did you only adjust by one number at a time?
Im guessing not because otherwise one number would be too slow and the next number too fast and I dont believe thats possible.
So, find a starting point, either too fast or too slow and only adjust by one number at a time in whichever direction you need to go.
The speed should change accordingly and eventually youll hit the right number for the current beans, their age and the humidity on the day.
That should work providing all other things remain unchanged. e.g. amount of coffee and tamp pressure.
I hope you arent using the single basket because I hear that its almost impossible to use.