Is the shock on the 620 even adjustable? I don't know... if it is, you should have to collets on the top of the shock. One's a locking ring, the other one adjusts the preload. A key might've come with the toolkit, or else you can get one for a few bucks from your Ducati dealer. Loosen the top ring, then turn the lower ring to compress the spring until you achieve the preload you desire. On my S4 it's a huge pain to get in there with the tool, brace yourself for some knuckle scraping. It was much easier on my R6, where I'd simply add a click of preload for passenger operation.
I'm only 170lbs and would run one click from max preload on my R6, I don't recall off the top of my head but I suspect that I'm running my Monster shock pretty close to max preload as well. Get a new spring or upgrade the shock as soon as the funds allow it.
-R.