I have an '84 300CD. My gauge reads empty at 16 gallons consumed but my low light does not come on until about 19 gallons used. When I bought my car I had to clean algae out of the tank and also the sending unit so I've had mine apart. There are two circuits in the sending unit, one for the gauge and one for the light. For the light, as the float bottoms out, contacts on the bottom of it complete the circuit for the low light. There is not much adjustment available and you run the risk of breaking the fine resistor wire for the gauge. So when my gauge reads empty I have about 100 miles left and when the light comes on I'm down to 50. Here is just one posting of many about taking the sending unit apart.
http://www.peachparts.com/shopforum/diesel-discussion/54583-sending-units-out%3B-how-do-i-take-apart.html#post324699