Reflecting the last layer can be done in one of two ways: 1) Over the x axis 2) Over the y axis
This graphic shows how the x and y axes are laid out on the last layer.  
This graphic is symmetric about the x axis. 

This graphic is symmetric about the y axis. 

On the x axis reflection, the B and F faces are the only two faces that reflect off of themselves. This means that F reflects to B', F' to B, B to F', and B' to F. All other faces become their own opposites. This means R reflects to R' and so on. 

On the y axis reflection, the R and L faces are the only two faces that reflect off of themselves. This means that R reflects to L', R' to L, L to R', and L' to R. All other faces become their own opposites. This means U reflects to U' and so on. 
From Table  Over xaxis  Over yaxis 
L' B L U L' B' L B U' B'  L F' L' U' L F L' F' U F  R B' R' U' R B R' B' U B 
From Table  Over xaxis  Over yaxis 
B'U R'F R B R'F'R U B'U² B U  F U'R B'R'F'R B R'U'F U² F'U'  B U'L F'L'B'L F L'U'B U² B'U' 
Note that it does not matter which reflection is chosen because they are identical. If you reflect over the xaxis and then turn the cube in your hand so that the back face becomes the front face (180 degree turn of the entire cube), you will have the same thing as if you had reflected over the yaxis.