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On an 8 inch f5 Newt a guide scope is fine, don't complicate the setup un-necessiarily. What coma corrector are you using, if it is the Baader MPCC then spacing is always 55mm. With my ASI1600MM-C and my Starlight Express 36mm filter wheel the combo takes up 22.5mm of space (The camera screws right to the filter wheel housing), so simple math from there: 55mm - 22.5mm = 32.5mm of spacing required. Don't forget the 3:1 Rule in relation to focal length effect of having filters or other glass in the light path to the sensor. A typical filter of 2mm, and the 1mm sensor compartment cover on the camera = 3mm, thus you extend your spacing by an additional 1mm.
I use an ASI290Mini as a guide camera on a ZWO 60mm guidescope, works great on all my scopes, with the MN190 f5.3 being very close to your Newt.
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