How can I calculate the maximum range of an aircraft descending with power (not gliding)?
You can use the lift to drag ratio (L/D) to calculate for just about any stage of flight if you compensate it properly. Assuming your descent is steady-state, lift is equal to weight, using the L/D you can then determine drag. Next take drag and subtract the thrust produced by the engine to get the amount of excess drag need to be covered by gravity. You glide ratio will be lift divided by excess drag. This makes for a simple calculation of your variables remain constant, however you will need to use an integral if you want to compensate for changes such as differences in engine power at different altitudes, and reduction in weight as fuel is burned.