How much incoming solar radiation is initially reflected off earth's surface?
That depends a lot on the location and material at the surface of the Earth. Generally speak it averages out to a little over 10%.
The albedo, or reflectivity, of the Earth depends greatly on what is at the surface. Ice and snow have high albedo but something like asphalt has low albedo. To complicate matters the angle of the sun's ray affect the albedo of several surfaces. For example, water can have an albedo anywhere from almost zero to 1.00 (completely reflective like a mirror) depending entirely on the angle that the sun's rays hit it.
In the Earth's Energy Budget, the average albedo of the surface of the Earth is about 10%. The following diagrams shows you that of the original 100% of the solar energy that hits the planet 52% of it actually reaches the Earth, 46% is absorbed and 6% is reflected. 6 is a little more than 10 percent of 52.