I think it's a pretty straight forward relationship. If it takes 2.5 oz say 24 inches from the balance point to make the plane sit level, it will take 5 oz 12 inches from the balance point to make it sit level.

moment=weight X distance.

If you are talking about moving items (rx, battery etc) around then it's a bit more complex. I THINK you'd have to weigh whatever you're moving, find the difference in distance from the old location to the new location and the multiply to find the new moment.

In the above example 2.5 X 24=60 ounce-inches Lets say your battery pack is mounted 1.5 inches behind the balance point and weighs 8 ounces. It's moment is 12 ounce-inches. To move the battery pack to get it to balance...(60+12)/8=9 inches behind the balance point.

Oh, man am I ever getting a headache!