Is there a better way to do masks?

I'd say I'm beginner/intermediate, which is to say there are still areas of Darktable that I'm pretty sure I don't even know that I don't know..

How would you go about creating a mask for the towers?

I took the shot of these concrete pillars against a bright cloudy sky, and they came out underexposed. I figured the way to correct it would be to make a mask so I could increase exposure only in the masked area and not affect the sky. Drawing the mask was quick - however, it took foreeeeever to make minute adjustments to ensure it was on the exact boundary between concrete and sky. Each time I move a point Darktable redoes all previous adjustments the image, so there was quite a lot of waiting involved. And as you can see, if you zoom in there are still areas next to the column where the sky was included and looks white.

I suppose alternatives to drawing a mask would be to automatically generate a mask based on color (but in this case the sky and concrete are close) or maybe based on brightness (but again somewhat similar).

The Two Towers