Your task is tricky. You really need to know Fortran well. You have to be careful about how similar/different Fortran does calculations and what rules of truncation and rounding apply. Also, you need to be careful about the primitive types meaning in C# and Fortran.
Another approach from what has been suggestion (not necessarily a better one, it is just another one):
A - Consider re-writing the code in C# based on the business knowledge and the function, use the Fortran code just as a reference
B -Consider using a commercial tool that does the conversion job - Example: DataTek
If the routine represents a standard function or a function that you can buy a ready made dll for (such as numerical integration), use the standard function or the commercial product instead of manual translation, and your problem is solved.
If the above does not cut it, answer this question:
Do I need to optimize the code or just make it run. In other words, what is the business value of spending 500 hours to make the code better?
if there is no value in optimization, translate the code line by line and you are done.
If this is still not good, then:
0-Convert the Fortran code line by line into C# (or use Fortan CLR)
1-Do a quick test make sure it runs
2-Use a re-factoring (commercial tools are available) to help you write the code in a more optimized fashion.