How in the world do I calculate my GPA at my school that doesn’t have them?

Hi all, Brown student here. I’m wondering what’s the best way to calculate my GPA since my transcript/school does not have them. We only have A, B, C, S (Satisfactory), and then S* (Mandatory pass no pass). If we get 90% or above in our S classes, we get something called “Distinction.” In our internal GPA, which is only calculated for honors, Distinction classes count as A’s and are therefore calculated in our GPA. Do I count those to fluff up my GPA or just use the letter grades? I’ve gotten mixed responses from my peers.