In the aircraft manuals there will be cruise performance charts. I'm just going to make numbers for examples, I've never flown a Duke and haven't even seen one in years. You might have charts for running the engines at 50%, 65% and 75% of the engine's rated power. The charts will either be graphs or tables of numbers. The charts will tell you at a given altitude what MAP will give you the percentage power you are looking for, your TAS at that altitude and your fuel burn. Without the charts, unless someone can give you some ballpark numbers to work with, you can't get too far in you preflight planning.