Okay... I have my source landscape clip, with blue sky sitting in top half of shot.

I put that in Video Track 1.

Then I make a black video clip and place it in Track 2.

I apply a Ramp (blck->white from top to bottom) to track 2.

Then I go to the landscape clip in Track 1 and apply Calculations effect to it. I set the "Second Source 2nd Layer" to Video 2 and set "Second Layer Opacity" to 100%.

I then set the blending mode to 'Overlay'.

PROBLEM:
This works correctly for the top half of the gradient/landscape, but the bottom half where the ramp turns to white is lightening the ground.

So it is effectively darkening the sky like I want, but it is brightening the ground. Why does it brighten the ground? It shouldn't. I know it doesn't do that in Photoshop at least. Shouldn't white in the ramp turn to transparent? If not, how can I make it so the white in the ramp is transparent on the final shot?

Thanks for any help!