What is the meaning of signs in electrode potentials?
Chapter 14 of Harris' Quantitative Chemical Analysis has a helpful section entitled, an intuitive way to think about cell potentials.  Here's the corresponding graphic:

 

In this case, the author is describing a Galvanic cell with cadmium as the anode and silver as the cathode.  The potentials of the two half-cells are known and marked on the line-graph.  Electrons always flow to the more positive electrode, which in this figure is towards the right.  The difference between the two half-cells is the cell potential.

In your case, you know the separation (-0.07 V) and one of two points (+0.34 V).  If we ignore the sign for the moment, we can use the line-graph to narrow the possibilities down to C or D, since each of these differ from +0.34 V by 0.07.  The question now is, which direction are the electrons flowing?  When determining the cell potential using this line graph, we subtract the right-most number from the left-most number in order to obtain a positive potential.  Since the cell potential in your case is negative, we must be doing the opposite.  Therefore, the reference electrode potential is +0.41 V (D).