Stack isn't too bad really for defining a frame's vertical dimension, and Reach would be ok if it was calculated at a fixed height above the BB. There is a good reason for not bringing the STA into the Reach measurement because it would just confuse things again. And I guess for "setback" you can just look at the STA, and figure out the sort of post you would need to get the saddle in the right spot.

But like I said earlier, there is really no substitute for determining the xy positions of your saddle and bars relative to the bottom bracket, and then calculating exactly what it would take to connect the dots. In fact a fairly simple graphical program would be a nice thing for every shop and frame manufacturer to have on hand for sizing... although I guess it might encourage customers to be excessively anal about every mm.