You can get an ohm reading from the sender. It's the Green/Black wire coming from the tank. Hook the ohmmeter to that wire and to ground. That will tell you what the tank sending unit is seeing.
Yesterday when I was messing around, I got a reading from the sender of 5 ohms, which just barely moved the needle from "E" on the gauge...as to be expected. However, when I put a stick in the tank, I had almost four inches of gas registering on the stick. That's about the amount that's in that smaller, lower section of the tank. I don't know how much gas that section holds. I also have an new sender (not original), so there may be a difference in the actual swing on the float. I have a feeling that the new sender doesn't dip down into that section very deep.
As far as checking the gauge accuracy itself, the MGA article is very good, but the TR's use a 0-90 ohm sender rather then the 0-70 ohm sender as cited by the MGA guru. To check the gauge you can use a few 47 ohm resistors (available at radio shack in a pack of five for a buck). In addition, you'll need a 12V battery or small 12V battery charger. Hook one lead from the battery to the terminal marked "B" on the back of the gauge, and the other to the gauge case...the stud in the middle works fine. The gauge is not polarity sensitive, so positive or negative ground will work. When you first hook the battery up, the needle will peg to full. Then you can use the 47 ohm resistors to see how well the gauge is working.
Hook the resistors from the other gauge terminal...the one marked "T" for tank to the ground stud.
Two 47 ohm resistors in series (about 90 ohms) will read as a full tank.
One 47 ohm resistor will read as 1/2 a tank.
Two 47 ohm resistors in parallel (about 24 ohms) will read as 1/4 tank.
Also, make sure that you have the gauge in the normal orientation during the tests.
Hope this helps...