Gut feeling would say remove half the tailwind. So that would lead to 50 knots indicated.
But for the absolute correct response you've got to know the exact speed/fuel consumption curve of the engine/propellor/airframe. Because 50 knots indicated may, for all you know, be right above Vs and thus very wasteful.
The speed that doesn't change with wind is the best endurance speed since that speed is simply the speed of the lowest drag. But best range speed...
