I wouldnt worry about the clumping too much or the setting on your particular Rocky..... there is a lot of variation in actual zero on these machines, usually in the -ve region. Mine for example zeros in at minus 3.
Clumping can be caused by a number of factors other than how fine the grind setting is, e.g. higher than usual humidity, the oil content of the beans, and the list goes on... so I wouldnt worry about it too much. The main thing is that you are getting good distribution in the basket, pours in the right flow rate of 25-30 seconds, there is plenty of of fine, viscous crema and you are happy with the resulting flavour. All else is superfluous ;),