The percentage difference in range impact rises rapidly with increasing mph.
So, there is not a specific percentage of impact, it depends upon the cruising speed(s) involved.
At 45 mph, an open tonneau would have less than 10% impact to range.
Also, if the tonneau is open because the bed is full of cargo, the impact of an open tonneau will be less because the cargo will improve aerodynamics, especially if the cargo is just a tarped pile of beauty bark or the like, and not a tall, draggy shape.
In most cases the impact to range will not matter in any signifact way because people don't usually drive thousands of miles with cargo forcing the tonneau cover to remain open.