Redneck in training
Elite Member
Easy. Let say the tank is standing up. Put the square on top of the tank such a way that one leg is vertical parallel to the tank axis and the other horizontal crossing the tank approximately at the center. Put a mark close to the center on the tank and the square. Place the square on the opposite side of the tank and put a mark on the tank under the mark on the square. The center is exactly in between the marks on the tank. Repeat 90 deg. from the first measurement.