Mount a lens on the camera and measure the exposure for the scene with a known good light meter or camera.

Use a shroud to keep ambient light away from the light sensor. The light meter should only read light from the focusing screen of the camera.

Adjust VR1 until the metered exposure matches the correct exposure.

Repeat the last two steps at different exposure values for additional calibration points.

VR1 affects the speed of the shutter and the speed indicated in the viewfinder. They should change at the same rate.