The number on the Rocky can be fairly arbitrary as they all tend to true zero at varying points.
However, I would say that 15 seems a little high to me.
Which basket are you using? Single or double?
If it is the single, please remove it, clean it and put it in the box the machine came in never to be used again. It is more trouble than it is worth.
If you think the voulme/time looks about right, then I would suggest a finer grind and not trying to get too much in the basket.
I found a good dose measure for the Silvia was actually the long plastic scoop that came with the machine. It is roughly 7g per scoop and I found that 2 scoops just slightly overfilled each time made for a good starting point.
Hope that helps.