Okay.
US Pat 4,573,730. 4 Mar 1986
Chevrolet C-10 tested with different length half-tonneaus. 50% of bed length gives greatest benefit at ~0.045 Cd reduction. For other trucks it may vary- other lengths of 40-60% may suffice. The illustration shows not only the half-tonneau on the rear half of the bed, but a vertical piece at the front of the tonneau that boxes in that rear half.
Example not in the reading:
SAE Paper 881874, 7-10 Nov 1988
"Pickup truck drag reduction-- devices that reduce drag without limiting truck utility"
Figure 1
Full cover is rated at 0.030 Cd reduction.