When Mike (Boy His Car) and I designed the expanded dough calculating tool we did not attempt to provide for pans with sloping sides. We did that for the deep-dish dough calculating tool but that was because the dough is pushed up the sides of the pans by specific amounts, and the numbers would be different for the two types of pans (round) that are commonly used to make the deep-dish style of pizza.
The practice I have seen among manufacturers of sloping sided pans is that the pans are rated by the top dimensions of the pans, not the bottom dimensions of the pans. The pans that Buddy's uses, including the 8" x 10" pan, are sloping sided pans. We don't know the weight of the dough ball that Buddy's uses for the 8" x 10" pan, so we can't do any calculations to determine the thickness factor they use. If we knew the dough ball weight, we wouldn't even have to know the thickness factor. We would only need to know that if we wanted or needed to use different pan sizes.
In your case, if the differences between the top and bottom dimensions are slight, then I wouldn't worry about it too much since the differences in the ingredients quantities are likely to be minimal. If, however, the top and bottom dimensions are materially different, then you can average the top and bottom dimensions and use those averages in the expanded dough calculating tool. If we ever learn how much dough Buddy's uses for one of its pans, we will be able to correlate it with one of the sets of dimensions (top or bottom) and use that as the standard, even for other sizes of pans.