I think the guage is more of a general guidline for how much you have in the tank. Once it gets down to the 1/4 and 1/8 area, the slight variation in meter movement represents at least a gallon, I'd say.

Also, remember that at night, your tank cools, and if there is humidity in the air in the tank, you'll get condensation, and, oh geez, WATER in the tank. After 20 years of that....yeah.

I guess we all need to fill up our tanks at the end of the day. I'm not going to, though.