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Hi Peter,
I also have a small Riccardi (there are two versions) and spacing and orthogonality need to be very accurate. Having said that, I think Riccardi works best when used with slow long focal length refractors. My 105mm f/6 gets reduced to f/4.5, and even with a small ICX814 stars are slightly but noticeably elongated in the corners.
First thing to check would be whether stars in all four corners are symmetrically / equally elongated away from the centre. That would indicate accurate orthogonality. Then you could add a 1 or 2mm spacer and see if star shapes improve or get worse. Unfortunately, it takes time and patience to fine-tune a 0.75x reducer.
As Rick mentioned, there may be often tweaking required because in reality focal length of each scope may vary a bit, position of the sensor in a camera can vary slightly from unit to unit and each Riccardi reducer has some variation for sure (these are assembled in Taiwan I believe).
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