Math to scale a map based on two finger pinch?

[rygo6]

I am having trouble figuring this out.

Lets say I have two fingers, and they raycast to project their position on the surface of the map. So then I have two world positions on the map. I want to then pinch to zoom, so as these two world positions on the map get closer, the map zooms out. BUT the condition is I want the map to stay locked exactly on those two world positions as they move. The current 'pinch to zoom' example just zooms at an arbitrary rate in relation to the change in pinch. How do I calculate the exact change to the zoom value so the map zooms so as to appear locked to the two world positions on it's surface projected from the fingers?