If the gauge is out of the car you can use a battery, test leads, a ruler, the vise on a work bench, a small board and a C clamp. Test leads to sender, battery and gauge, board to vise, sender to board with C clamp, ruler to tank, measure depth from flange to bottom, subtract 1/2", measure flange of sender to bottom float, add a little, move the sender, watch the gauge, bend sender float arm as required.

If the gauge is at the body shop with the car I question that it will work with ohm values. Mine were a bit different than those Steve cites, even though I think both he and I were using 12 volts. I think you are using 6 volts. It was fun to figure it out on the bench with my Stewart Warnere sender, Ron's article and some resisters. A side benefit is that you will better understand the "ground" problem.

Of course you won't be sure of your answer until you move this from the bench to the car (which I have not), but I think the measurement can be done as described above.

Keith
444 Project, Moline, IL