Also, do not forget the influence of headwind or tailwind.
On the DH8D in still air and more so in a tailwind, FL250 is not only maximum, but also a good bet for optimum range. Reduce torque to long range cruise and just trundle along. Facing a strong headwind, you may well fare much better with rated cruise power and at a lower level: while you burn more fuel per unit of time, you will often burn much less per mile than higher up.