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Adam,
I have had similar queries about the calculated focal length being different from what is written on the scope. From what I undersand, the software gets the pixel size from the driver for your camera (read that bit in the help file). I presume it then calculates the focal length backwards from that using the pixel size and position of stars on the image. My 2000mm SCT is 2222mm according to plate solving from Maxim.
The amount of "reduction" your reducer provides is dependant on the distance between the reducer and the imaging plane. Small changes can have significant effects on that reduction and hence the effective focal length.
Under the observatory button on Maxim there is a catalogue and sky view - apparently you can overlay your image onto that sky view. I guess it might help with composition of shots.
Other cool thing about having plate solving working is coming back to an image on a subsequent night. You take an image, plate solve that and sync your mount. Then you open up a previous image, plate solve it and tell your mount to "goto" the centre of that image. Now you can shoot the same field of view night after night. The plate solve also gives you an "angle" in case you've rotated the camera for composition - therefore you can match that angle again on subsequent nights.
DT
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