Stall speed is related to angle of attack and airspeed, but there are other factors, including wing loading. Dropping flaps alters the angle of attack of the wing without having to pitch the nose up; when the trailing edge of the wing lowers, the mean chord line (a line drawn through the wing from the center of the leading edge to the trailing edge) is angled more sharply to the relative wind, increasing angle of attack along the span of the flap. It does nothing to increase lift on the wingtip beyond the end of the flap, so flap performance on your plane will depend to a great degree on the "flap span" vs. the total wingspan. Lift may increase with partial flaps, but beware of higher settings if you have relatively narrow (span) flaps, because the tips beyond can stall if you get slow enough, even though the inner portion of the wing is still generating adequate lift, overall lift may not be enough to maintain flight and the plane will suddenly drop. Take a look sometime at some of the STOL planes, such as the Pilatus Porter or the Helio Courier/Stallion, and how much of the total wingspan is devoted to flaps. These aircraft can significantly increase total lift over 90% of their wingspan both for takeoff/landing and slow flight. The Courier, for instance has a max gross takeoff weight of over 3600 lbs but can fly at 27 mph (cruise is 140) and land in less than 100 ft. Compare that to a Super Cub, which weighs slightly over 1200 lbs, but stalls(with flaps) at about 35 mph. This is a rough comparison, but you get the idea. (The Courier also has self-deploying leading edge slats, which also improve lift)