I finally found an old archived thread that discusses a reasonable workaround. http://forums.adobe.com/message/4905560 Clicking on the + symbol next to one of the sliders clears the values for all the others but the one you chose to click + for. That allows me to pick a starting adjustment and clear all the others.
Now I would not expect the + slider button to work that way. Another oddball non-standard interface quirk to assimilate.
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When you first open the tool, click on one of the circles on the left side of the sliders. This will reset all the sliders except it will set the exposure a bit under, but that's okay.
Yep, that's the circle with the plus sign mentioned above. The minus button on the left that you mention does the same thing. It's not behavior that I'd expect of plus and minus buttons, but at least there's something that helps.