I watch the fuel drop and I am roughly getting 130-160 every quarter tank. So around 280-320 by halfway and then the fuel seems to drop faster and I end up with somewhere between 480-525 for the tank.
If you want to be reasonably legitimate with this, please stop giving rough estimates and just do things the right way. The gauge will tend to drop faster on second half of tank because it is setup to do so. Without going into a longer explanation it makes people feel like they are getting better gas mileage.
1) Reset the trip odometer (as you said you do). Fill up tank til it clicks (I don't recommend overfilling, will kinda skew data).
2) Drive car until you go to fill up.
3) Write down how many gallons went in, and how many miles your trip odometer was at.
4) Simple division for miles per gallon.
5) Compare results over multiple gas tanks.
6) Bonus for smartphones: record it every time through an app and look how it changes over time. This will be first winter for me doing that with this car, I'll let you know how mine does ('12 MT-sky sedan).