transform > scale from a pre-selected origin point...how ?

Hi

CS6

scaling two images that I wish to match up.

set uppet to 50% opacity, edit>transform, move gunsight to where I have matched the photos so as to keep that area matched.

despite moving the gunsight icon that appears in middle of transform box to where I want the scaling to happen from, it doesnt act as the origin of the scaling. I lose my already matched part of the image.

I thought this icon could be used as scaling origin, apart from rotation origin.

How does one scale a layer from a specific origin ?

Better still, to be able to select points on one image and then their target locations on another underneath and have pshop rubber distort for best match !

That would be fantastic, so useful and a fundamental image manipulation tool. Afetr all these years of development these two requirements need to exist.

Merlin