I BELIEVE,( but could be wrong!) that the way it works is that the power reserve doesn't actually hit zero. As you get closer to zero, the accuracy of operation decreases, so I believe there is a level just above zero. After this level is reached, it waits for the watch to complete a minute cycle, and then disengages. It is pretty close to zero, but there is technically a tiny amount of fuel left in the tank, but you won't get to use it!
I might have also confused this with a GO movement, possibly the chronometer, but I believe that someone operates the mechanism as mentioned above, and am pretty sure it was either Lange, or GO, or both.
Looks great btw!!