Read your question before I went out today. Had to go down a steep hill with a 50 mph speed limit. I did not use the paddles, but had my foot off the accelerator and was able to maintain speed while the battery gauge was in the green regen area. I had the estimated mileage on the DII and it slowly increased the estimated mileage one tenth of a mile at a time until the hill leveled out and i needed accelerator to maintain speed.
FYI if you set the ACC speed on a descent the car will use regen to maintain the set speed, which is a nice thing that a regular ICE-powered car can't do.