A simple way to determine magnification is to measure the height of the in-focus subject field and divide that into the height of the focusing screen. Determining exposure with the camera's meter is probably as easy as any method. Rulers have been available (or can be calculated and printed) that give magnification and exposure compensation when they are focused on. The formula for exposure compensation (E) is E=(M+1) squared where M is the magnification ratio (from www.macrophotography.org).
Originally Posted by Jim Noel