So long as your seat height and setback are set correctly, then I would say you probably need shorten the stem by 20mm. One way to try this out before buying a new stem is get the stem used from a LBS or on eBay. For the price of a new stem you can buy a 100 and 110mm. The stem length is roughly correct when in the drops your: 1) shoulders are not hunched forward and 2) with your forearm parallel to the floor your upper arm to forearm angle is roughly 90 degrees.