my 300d guage would never go past 3/4 of a tank, and would consistently fluctuate a little bit, getting worse as the weather heated up (it's about 105F/41C here today). i would just go by the "fill it at 350 miles" method.

now, the car has been sold, but not before the odo was stuck permanently at 144300 (not actual mileage...this odo came from tristar pete mbz junkyard, and the actual mileage is about 350000).

it's a good thing that the guage works at least marginally, or the new owner would have to use a broom handle to check the level :-)