Uh, are not the "sending unit wire screw" and the "center screw that is insulated" one and the same? One wire to the ohmmeter should be hooked to the sender wire screw terminal (on outside side on tank) which by the way is insulated from the mounting plate. The other ohmmeter wire goes to the mounting plate, thus grounded to tank when mounted. When moving the float arm up/down you should see the ohmeter value move. Note...the variable resistor or rheostat is not voltage sensitive. It controls the amount of current flow through the gauge but that is beyond this post and can be quite a can-of-worms. Good luck
