like I said, the coordinates aren't the problem here, it's rather that the decal covers two meshes, Left/Right_VStab and Left/Right_rudder. (I do think that the meshnames are correct.) 
  
[Decal005] 
MeshName=Right_Vstab 
DecalLevel=3 
DecalFacing=RIGHT 
FilenameFormat=SU-27N/Grey Camo/D/LT 
Position=-5.9,2.2 
Rotation=0.0 
Scale=3.0 
DecalMaxLOD=3 
  
[Decal006] 
MeshName=Left_Vstab 
DecalLevel=3 
DecalFacing=LEFT 
FilenameFormat=SU-27N/Grey Camo/D/RT 
Position=-5.9,2.2 
Rotation=0.0 
Scale=3.0 
DecalMaxLOD=3 
  
[Decal007] 
MeshName=Right_Vstab 
DecalLevel=2 
DecalFacing=RIGHT 
FilenameFormat=SU-27N/Grey Camo/D/RTailnum 
Position=-5.8,1.7 
Rotation=0.0 
Scale=3.0 
DecalMaxLOD=3 
  
[Decal008] 
MeshName=Left_Vstab 
DecalLevel=2 
DecalFacing=LEFT 
FilenameFormat=SU-27N/Grey Camo/D/LTailnum 
Position=-5.8,1.7 
Rotation=0.0 
Scale=3.0 
DecalMaxLOD=3 
  
[Decal009] 
MeshName=Right_Rudder 
DecalLevel=3 
DecalFacing=RIGHT 
FilenameFormat=SU-27N/Grey Camo/D/LT 
Position=-5.9,2.2 
Rotation=0.0 
Scale=3.0 
DecalMaxLOD=3 
  
[Decal010] 
MeshName=Left_Rudder 
DecalLevel=3 
DecalFacing=LEFT 
FilenameFormat=SU-27N/Grey Camo/D/RT 
Position=-5.9,2.2 
Rotation=0.0 
Scale=3.0 
DecalMaxLOD=3 
  
[Decal011] 
MeshName=Right_Rudder 
DecalLevel=2 
DecalFacing=RIGHT 
FilenameFormat=SU-27N/Grey Camo/D/RTailnum 
Position=-5.8,1.7 
Rotation=0.0 
Scale=3.0 
DecalMaxLOD=3 
  
[Decal012] 
MeshName=Left_Rudder 
DecalLevel=2 
DecalFacing=LEFT 
FilenameFormat=SU-27N/Grey Camo/D/LTailnum 
Position=-5.8,1.7 
Rotation=0.0 
Scale=3.0 
DecalMaxLOD=3