The most I've ever put I've ever put in the NT has been 4.967 gallons. The needle was just barely above where it is when the switch is turned off.
I've noticed this, also. Right on at 1/2 tank mark.I agree with Phil about the shape of the tank being a key factor in how fast the gauge needle starts to drop. One thing I've noticed about the fuel gauge is its complete accuracy when it registers 1/2 tank. I filled up once at that point and it took 2.6
gals which is exactly half of full capacity of 5.2 gals.