The True Airspeed an aircraft achieves is a function of the power you set and the drag. The power setting you use is based on how much money you have - 55% if you can't stand the fuel bills (the Duke is a thirsty aircraft), 65% as a "standard" - 75% for "damn the bills, full speed ahead!"
For piston aircraft, pilots simply memorise the speed their aircraft "trues out" at. For a Duke at 65%, you are looking at 48 US gph (180 litres per hour) and 210 knots TAS @ FL200 or so. If you fly lower, the drag goes up in the thicker air, and the aircraft will thus lose about two knots per thousand feet (so at 10,000' you will be looking at about 190 knots, and the same fuel flow).
So - look at your wind forecast, choose the best level for wind (least headwind/most tailwind) work out your TAS at that level, add/subtract the tail/head wind to get the ground speed. Measure the distance you want to fly, divide that by your ground speed to get the time (in hours) and multiply that time by the fuel flow above to get the fuel required. Throw on a bit extra fuel, so your Mum won't worry about you not coming back, and Bob's your uncle.
