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My experience with flatteners is that they have a very shallow critical zone where they're effective. And it could be out quite a bit from the theoretically calculated distance. My refractor flattener and camera specs told me I needed 20mm spacer between flattener and my camera. After doing a lot of testing I found that it was actually 30mm! That's quite a difference.
Keep in mind that all of the distances/spacings they give you are plus/minus tolerances. Not sure if that can all add up to 10mm over such a short distance, though.
I have a spacer for my MPCC on my 10in newt as well. It's exactly as per the specs/numbers calculated, but it's still not quite right. I still need to test/tweak that spacing like I did with refractor. Just haven't gotten back to it yet.
What I'm getting at is that you may need to play with the spacings, say in 1mm increments both in and out of where you calculate to find the actual correct spacing.
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