By definition, the Standard Potential is calculated using the expression
#E^@#half-reaction potentials are tabulated reductions.
From the Table of Reduction Potentials, one should visualize the reaction with the more positive reduction potential as the reduction half-reaction and the reaction with the more negative reduction potential as the oxidation half-reaction.
Substituting the values given into the above equation give the Standard Potential for the combination of half-reactions selected.
For the half-reactions in this post...
Expressing the more-positive value,
and expressing the more-negative value,
Adding the two reactions such that charge transfer is equal between the two reaction gives the net oxidation-reduction reaction ...
#Co^o + F_2^o => Co^(+2) + 2F^(-) ; E^o = "+3.15 V"#