Your CofG varies and Something for from it will have a bigger effect at moving it it, Extra weight AT the CofG will not alter it. Anything ALTERED in the allowable CofG range won't take it Out of the allowable range. For ANY Particular gross wt the Moments added up must be withing 2 figures (aft and fwd) already worked out. There is ALSO a Graphical way of doing all this. I drew up one for the Citabria. That way you can put compartment limits on the chart.. It will show TO Wt . Wt for intended landing and a ZERO fuel wt and CofG and for the other fuel wts as well. Look at examples and work through each, till it makes sense. W&B is a 100%pass exam as is fuel management. nev