I use the gauge. Math is hard!
I can't even complain that I don't have a calculator handy, since there's one built in to the Nav system.
My driving pattern is too mixed to try to calculate the whole tank. That 36mpg running around town would completely negate the 52 I've managed on the highway. Let's face it: with a 500-mile cruising range, it takes a LONG time to use a whole tank (not that I'm complaining).