This is my GX680 exposure compensation chart. I suggest that you print a small copy of the chart and attach it to the camera back or another convenient location.
Using *any* lens, you will need to apply bellows compensation for an image-filling subject measuring 760mm (long side) or less. The chart works in conjunction with the camera's millimetre focusing scale. Lens focal length is in the left-most column; the column just to the right indicates where infinity focus will lie on the millimetre scale, and the following columns correspond to readings along the millimetre focusing scale. The upper-most row of boxes (each containing three figures) indicates 1) scale in proportion to life-size, 2) actual size of the area being photographed (if assumed to be a subject with little depth), and 3) exposure compensation to add to your handheld meter reading.
You'll find figures for the 50, 65, 100, 135, 210, and 300mm lenses. I've used these lenses because they are the ones that I have; if you have other focal lengths, you can determine approximate compensation values by choosing values that are intermediate to those that I've given here. Within each box that corresponds to distance on the millimetre scale, you will often find two figures. The upper figure is to be used when no extension rails are mounted on the camera; the figure in parenthesis (the bottom figure if two are indicated) applies when the 80mm extension rails are in use. I suggest highlighting the latter figures with a color marking so that you won't mistakenly read the wrong set of figures.
There is an additional small box following the main chart. This is a convenient means to quickly determine appropriate aperture for good DOF: the millimetre figure given will indicate the amount of front-to-back movement when focusing from the near focus point to the far focus point of your image (if you are using movements, apply them first), and the figure to the right will suggest the appropriate aperture to use. By the way, these figures will work for any camera with bellows focusing, not just the Fuji 680.
If you have any questions regarding the chart, feel free to ask me. The charts may be printed by clicking "Print" in the browser bar.