If you're REALLY fast with the calculator, and good at guessing angles, you can get a rough estimate of above ground altitude using trigonomtry. Then you'll have to use trig again to find the total range to the above-ground height of your aim point.
Essentially, you would doing everything that the german gun-control systems were doing, save for calculating speed and a precise aim point.
Personally, I think this is going to be like the 5"ers, but even more so due to the shell randomization: Art much more than a science.