For Ohms, or the top on the meter pic, or Ohmega, all the way to the left is infinity, and the right zero Ohms or zero resistance. Get lazy. a DVM is easier to read.
Pop the sender out of the tank and have a look, measure into and out of it or take measurements in between.
Regardless, depending on fuel level or float level the needle and voltage at the gauge should vary to ground.
Welcome to automotive electrics. Think aobut working on a (today) modern vehicle w/ 3 miles of wire and 10 little computers running things when they're 40 years old. Yikes! Then again at that time an EE will be 4th grade.
