I am only repeating what the standard says. I imagine it would be difficult and take more pages and whatnot to address each bird by the angle of its back and then compute the angle of the tail. Not all birds are flat backed, many of them have varying degrees of sloping. Again, it's not my determination but rather that of the standard that says it is measured from horizontal.The Sussex is always said to have a flat level back. Thus measuring the angle of the tail in a Sussex from the angle of the back. Anyway, a bird's tail is connected to its body, not the ground. If one is measuring tail angle to help determine symmetry in the bird, why would one use a measurement that is not intrinsic to the bird? i.e. the ground on which it stands. Using the ground as a measuring tool may help to create symmetry in a landscape but not within the bird itself.
Karen