First of all I'd try to figure out if its really soft, or maybe just a mis-match of expectations?
What lens is it?
IMHO if you need a caliberation, I'd only let Nikon do this.. Fixation are good, but last I heard they couldn't do a "calibration" as this needs software they don't have. I could be wrong, but chances are if its an AF-S lens, it will need to go to Nikon UK for tuning.