there's a litle handle or notch, just like there is on an SR3, or on an f900, or any other 'serious' camera, on the top of the adapter box that houses the gg.
i opened up the brevis to get an idea of just where the ground glass was, then marked that spot with a piece of yellow paper tape on the outside of the unit. not exact, but within a few millimeters. close enough for even very shallow dof.
