question about roof box, aerodynamics

I installed the Thule rack according to specs, which places the crossbars 25" apart. When I line up the holes on the cargo box, the farthest back I can get it is like this:



I read, however, that having the box further back is better for aerodynamics. I don't want to change the positioning of the bars, so should I drill an extra set of holes in the box? That would put it about 3" or so back, thus reducing that dead area in the front that looks like it'll trap a lot of air.
Or would the rear overhang be just as bad for the aerodynamics?

 

It won't be too heavy of a load, & the heaviest stuff will be between the bars. Mainly just tent, thermarests, bedding, shoes, jackets.

Driving wise, the only concerns I have are crosswinds in the plains states & the hills in Utah. I remember on my last trip those being a bit of a challenge even without the box.

 

I'm really curious what kind of MPG hit I'm going to take. I'm at a steady 46 now.