Lightroom :: Specify Begin And End Exposure Adjustment On Gradient Tool?

Is there a way to specify the begin and end exposure adjustment on the gradient tool?
 
What I really want to do is apply an even exposure adjustment to the bottom section of an image. There's a hard line where I want the adjustment to begin, so I don't really need a gradient. I know I can do this with the adjustment, but that requires passing the brush over a large section of the image and, of course, being very careful in lining the edge of the brush up with the edge of what I want to lighten. I'd like to be able to put a line across the image and say "lighten everything below this line" by x amount.