I completely agree with Mr. Scudder above. For the extremely non-technical minded, half box speed for most films means overexposure of one stop. If we err on the side of overexposing in order to make sure we've placed generous shadow detail in Zone III (or even IV, as some would have it), then we've in effect overexposed by a stop, functionally the same as halving the box speed. Same end result.
That's the way my fragile brain sees it, anyway. :-)