MOD HELP/ADVICE NEEDED
 

When sailing to US east coast type VII boats increased their range by filling one of their drinking water tanks with fuel. I sail with NYGM which means type VII boat can sail there and back; barely. With h.sie's U-tanker mod I can get one quarter of my full fuel capacity every two weeks. But historically type VII boats only refueled once before their return journey.

So my questions are:
1) What file and what part of it I need to change?
2) And how much I should increase it? (1-2 metric tons??)