How do I calculate a GPA, if this is requested by some external party (employer, school, internships, etc.)?
This is straightforward computation, as follows:
Do not count Ss.
However, if you have S- with-distinction, you can use this same as an A=4.
Using the grades you have, add up the numbers as per above, and take an average. You can include courses taken at other school for college credit (study abroad or away or summer courses), too.
Remember, this is a self-computed figure, since there is no official GPA reported by Brown. Your transcript will substantiate the figure you calculate.