I have not worked on the parts and so my answer is suspect but the way I read it the instructions are:

1) Remove the sending unit from the tank.
2) Read the resistance with the float all the way down.
3) Read the resistance with the float all of the way up.

I am assuming that the float rotates a part that turns 180 degrees between empty and full, hence the 180 degrees reference.

BTW maybe we should average our gauges. I have the exact opposite problem. The light does not turn off. Since the fuel gauge works fine I was assuming the problem is in the gauge itself.