How to create a variable-opacity mask (by adjusting a slider)

I am contrast-correcting B&W negatives from the 1950s that are seriously overexposed. If they weren't of historic interest to Tasmanian bushwalkers, I'd be binning them. As it is, I'm trying to correct them.

The overexposure, after an initial correction with a Curves 1 layer (for maximum contrast) and then a Curves 2 layer (for overall contrast) renders the shadows to solid black. I correct the blacks by selecting them (from a layer above), so that I select just those levels that have been rendered black. I feather by a certain amount, and with the selection still active, I create a Hide Selection layer mask for Curves 2. The result is that the black area is now at the original overexposed level and looks out of place. Filling the mask with a shade of gray alters the amount that the shadows are darkened to try and match the surrounds.

The method works well enough for the poor images, but it is time consuming because I have to run through the fill process several times before I achieve a suitable effect. What I'd like is to be able to adjust the opacity of the mask with some kind of slider so that I can quickly choose a suitable value for the density of the mask.

Anyone know how to do that?

A sample file can be downloaded here. Note there are several other adjustment layers not in play for this image.