Since the two gravitational forces from the mountain is sideways (relative to "down"), the force from the string must be diagonal. Now you just need to know the mass and distance to the mountain.

The science of calibration dates to 1949, when the U.S. Air Force was figuring out how many G's a human being could endure. That is, they wanted to know how to calculate gravitational force, G-force, ...

Theoretical physicists from the University of Basel calculated signals from a long-lost cosmological phenomena known as ‘oscillons,’ gravitational wave sources from just after the Big Bang.