You can avoid most of the math by using...

#include "transforms.inc"
#macro ShowGuide(Position,AtX,AtY,AtZ, BeamSize)
union {
   //centre of beam
   cylinder{ <0,0,0>,<0,2,0>, 0.005
      pigment {color Yellow}
   }
   //to define top edge of beam in Y
   cylinder{ <0,0,0>,<0,2,0>,0.005
      pigment {color Red}
      rotate< -BeamSize,0,0>
   }
   //to define bottom edge of beam in Y
   cylinder{ <0,0,0>,<0,2,0>,0.005
      pigment {color Green}
      rotate<  BeamSize,0,0>
   }
   #local p = <Position,1.2,-0.6>;
   #local pa = <AtX,AtY+0.3,AtZ>;
   #local ny = vnormalize(pa-p);// new up vector
   Point_At_Trans(ny)
   translate p
}
#end

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