Mark Yes it lowers the stall speed due to the reduced drag of the aircraft in ground effect as compared to a measured stall speed in flight say at 2000'.
I stand corrected if someone finds it in print from a reputable source but I don't believe there is a set amount. It has a great deal to do with wingspan, proximity to the ground, the angle of attack it stalls at out of ground effect, etc... I would think that it is approximately 3 to 5 mph depending on the KR. This is best guess because I am not looking there when I am landing. Kinda hard to tell because your sink rate is beginning to increase just before, so you are flaring to control that for a 3 point and the wheel landing you just let it come on above the stall speed. Colin
