You can add these settings to your Skyrim.ini and tweak them until you have the camera position to your liking.
[Camera]
fOverShoulderCombatAddZ=0.0000
fOverShoulderCombatAddY=0.0000
fOverShoulderCombatAddX=0.0000
fOverShoulderCombatPosZ=0.0000
fOverShoulderCombatPosY=0.0000
fOverShoulderCombatPosX=0.0000
fOverShoulderPosY=0.0000
fOverShoulderPosX=0.0000
fOverShoulderHorseAddY=13.0000
fOverShoulderHorsePosZ=0.0000
fOverShoulderHorsePosX=0.0000