Regardless of the tanks position when balancing the plane for your first round of flights. Lets say your "maiden flights" until you get used to the plane, go with the 4 7/8" recomendation. Once you got your wings on this plane start to experiment with the CG. The plane, when balanced with the tank empty, starts your flight with a slightly nose heavy plane as you burn off fuel and when you are prepared to land empty, the plane is now properly balanced. At 4 7/8" with the tank empty or full, 4 7/8" is nose heavy for your introduction to this model. When a tank is any where on the model other than the CG it acts as a ballast that changes with volume, therefore resulting in slight changes in the CG. With the tank on the CG it is now just a load effecting only the over all weight of the plane not affecting the CG at all. The plane just gets lighter as it burns fuel. On my last one I worked my way up to 5 1/2" CG. Now on this model with the tank on the CG my balance point will still be 5 1/2" Joe