Getting the airplane to balance by moving heavier items to the extremes of the fuselage is the first consideration. But just as important is minimizing the moment of inertia by keeping the mass of components as close to the CG as possible. This effects how well an aircraft can respond to control inputs.

As far as calculating the amount of distance you have to place an item, you only need to know the total weight, and distance you want to move the CG from it's current position. Multiply that distance by total weight, and then divide that figure by the weight of the object you are moving. That gives the distance it need to move.