I like your formula. It probably is pretty close to what FFG starts with, but it is not where it ends too. But at least, it gives a good starting point when designing your own ships. I will definitely open this thread again when I feel like doing some play design on my own.
I do not see a point why you should not be able to use your scheme with X-Wing and Y-Wing as the base ship except that they are overcosted when compared to the B-Wing/Z-95, but you could just add another factor for that too, so the " the X-Wing and Y-Wing predate the Z-95 and B-Wing and so cannot be priced based upon their attributes " argument is incredibly stupid. I am not sure how you imagine mathmatics to work.
Another point on how math works: IF FFG had a formula, this formula could be total bull as well, since it is just a model. As FFG's model probably is not perfect (because there obviously are good and bad ships) it does not mean, that finding a formula is that is different from FFGs is a bad thing, the formula you came up with might be even better than FFG's.