There is more to this than getting the color right. 100g of 80 L and 200g of 40 L may give the same color, but they won't give the same flavor. Having said that, I have made similar beers, and I find that the Munich and Victory are the most important for flavor, the others are mostly for color and have a minor impact on flavor. How important is it to you that the beer tastes exactly like Fat Tire? When I first started brewing I tried to clone my favorite beers, but I have found that there are wide variations in clone recipes (as you found) and even then you get different results based on your process. Now I just try to make beers that taste good, and I have greatly increased the amount of Munich in my amber ale (5 lbs 2-row, 4 lbs Munich and 1 lb Victory along with some C40, carapils and chocolate) because I really like the way it tastes.
I would say go with whatever you have already purchased and make choices that are convenient and affordable for you. Don't knock yourself out trying to get some particular grain because someone else's recipe calls for it. Try the brew and see how you like it. If it isn't dark enough or malty enough, then make some changes the next time. Your beer will probably taste great even if it isn't exactly the same as Fat Tire.
--GF