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I have done the same calcs for my 10" gso dob.
The supplied secondary is 63mm but the holder thats the mirror sits in has a thickness that adds to central obstruction giving a total of about 26% CO.
One can go down a mirror size or more remembering two things:
1: Imperfections in secondary mirrors are more likely to be an issue at the outside edge, meaning a smaller secondary has to be of good quality because you will be using more of it.
2: You are sacrificing illumination at the edge of field at low power. If you dont own a 31nagler or similar low power wide field you can afford to go a bit smaller.
For the 10" , one can go down to a 2.14" inch secondary without loosing too much illuminated field.
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