The best approach (IMHO) is the following:
First you figure out the better save progression of both sides for every level (for each save type).
1st Fighter | Rogue -> good Fort, good Ref, poor Will
2nd Fighter | Sorcerer -> good Fort, poor Ref, good Will
3rd Fighter | Sorcerer -> good Fort, poor Ref, good Will
...
Then (for each save type) you count how many levels are good and how many levels are poor.
i.e. for the 3 levels above: Fort 3 good 0 poor, Ref 1 good 2 poor, Will 2 good 1 poor
(this essentially means, that a Ftr3|Sor3 has the same saves as a Brb1/Ftr2|Sor3, unlike normal multiclassing)
Then you look up what a class with a good save and a poor save has at those levels and add them together (again for each save type).
i.e. Fort +3, Ref +2, Will +3
(same result that you got here, but I think the calculation is different, which would show at higher levels)
Another alternative is, that you calculate the complete saves for each side and simply choose the better, but the above seems more in line with the spirit of the Gestalt rules, I believe.
Bye
Thanee