FYI, firing at 18-20 Km, you would never, ever see the TGT. The TGT COODRS and the Howitzer position are all computed in a software, and the result of this is a Bearing, an Elevation and the charge (how many charge bag needed) for each gun. The gun det member apply those numbers on the gun and fire away.
Now for the sight, what you look at if you were a gun servant, is a close or far reference point.
Pitbull
Edit;
The only time you would see the TGT thru the gun sight is if you're doing "Direct Fire", in that case it's self defense and your day as gone from bad to shi**y. Your basically trying to beat an MBT to it's own game. good luck... ;)