OK, I see. But there is something fishy here, IMHO just because these numbers add up doesn't mean they are correct. Unfortunately my thermodynamics is so rusty I don't see what is wrong with this approach, but it is not that difficult to show that something _is_ wrong.
Imagine you have a piece of iron in an acid solution. For the sake of simplicity let's assume activities of all substances involved are 1, so we can deal with standard potentials. Iron dissolves and hydrogen evolves. In such solution, as suggested by the potential given, iron gets oxidized to Fe3+. So, what we do have now? H+ and H2 - that means half cell of potential 0, and Fe3+. But if you look at standard potentials table Fe3+ can't exist in such mixture, with its standard potential at 0.77 V it will immediately oxidize hydrogen to H+ and get reduced to Fe2+. So there will be no Fe3+ in the solution (well, some traces), only Fe2+.
I have a feeling that potential given for Fe/Fe3+ is calculated using approach similar to the one you have used to check the numbers (so in fact you have just reversed the procedure getting back to the original data - no wonder numbers add up). There is something wrong this approach, as it gives values that won't survive in solution.