I'd say it would be measured at the input of the gear train....IE: the motor shaft. Assuming the load (or ball screw) were held stationary, the measurement would be the angle of rotation of the input device.
RC
Gearbox backlash is the angular rotation in degrees, minutes, and seconds of the amount of play in a shaft. What that means is; if you were rotating a shaft in one direction, pushing a load forward, then you reversed the rotation, the amount of free play before the load starts to move in the opposite direction is the backlash. It is measured in degrees, or parts of a degree (there are 60 minutes in a degree, and 60 seconds in a minute). The best explanation I ever heard for arc-minutes, or arc-seconds, is this.... If an airplane left New York City heading in a straight line for Los Angeles and was off by ONE-ARC-SECOND, it would miss the center of the runway in L.A. by 12 feet. So over a distance of 3000 miles, you would be off by 12 feet. If you were off by 1 ARC-MINUTE, you would miss the runway by 60 times 12, or 720 feet. (about 2 football fields).