Dear all, AoA

I need a little guidance from experienced people here. I aim for Foreign Services of Pakistan and planning to attempt CSS-2019 for it. Please guide for the following:

1- What is the closing merit number for FSP generally?
2- How much written score is good enough for it?
3- Check my optional subject combination and suggest me for minimum effort and max ouput:

International Relations. (200)
International Law. (100)
Islamic History and Culture. (100) [or suggest if USA history is better]
Public Administration. (100) [or suggest if any other is better]

This is 500. Suggest me a 100 mark course. My background is Electrical Engineering.

Thank you!