How is child support determined if I am in the military?

I am currently deployed and going through a divorce with one child. I was told that I will be paying child support since I am here and they are there and that I would pay 20% of what make for child support. So I was wondering what part of my pay do they determine that from is it base, bah and bas or just base. I also was wondering how is bah determined after the divorce for me? We are going for joint custody but I'm not quiet sure as far who is getting primary.