GetRasterProperties_management (in_raster, "STD", {band_index})gives you the Standard Deviation.GetRasterProperties_management (in_raster, "MEAN", {band_index})gives you the mean.Similarly, MINIMUM and MAXIMUM returns the range.The first Standard Deviation should be centered on the Mean: half above and half below.The other breaks are full StdDev steps in each direction, to the end of the range.