If you like 6x7cm because of its similarity to 4x5", the actual image made by the Fuji 690, when scaled up, would be 4x5.8" so it is not as far off as one would think.
The finder lines show a 92% view. So, if you use the top and bottom of the viewfinder image and the two sides of the frame lines to compose, you will have a rough approximation.
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The viewfinder on any camera is just an approximation anyway. If you are serious about composition you might look into something like a Horseman VH which has ground glass composing ($250 ebay).