Around 30-40 pg/mL is what I have been told by most experienced guys. My levels happened to end up at 30 but from experimenting with more AI or less AI I felt that dose and at that range made me feel best so I went for it (didn't do labs for those other tests just went by feel).

Cardio is good not just for staying lean but for cardiovascular health. In your case you're pretty light I wouldn't use it to keep that weight, I'd eat a lot more than I burned and made sure I gained some weight and always had energy for training. What's your diet like?