This is what I do:
For my favorite planes I have recorded the fuel burn for wep, military, normal, and max cruise settings. Then, especially for bombers, I have calculated an average true airspeed for a climb to say, for B-17's, 26,000. I also have an average climb fpm for that distance as well. So, I know that at 50% fuel, its going to take X amount of time to climb, over a distance of X amount of miles. That's going to be at full military power, so I take that time, mulitply it by the burn rate, and I know what it will take for the climb. Then, I look at the remaining distance between the end of the climb and the target. Mulitiply that distance by the normal power setting fuel burn. Figure out an approximate landing field. Multiply distance between target and landing field, mulitply that time normal fuel burn. Add up all those gallon amounts and you've got a pretty good fuel consumption reading for that mission. If you really wanted to geek out on it, pick your decent point for approach back at the landing field, and do a fuel required for descent and landing calc.