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It takes a while to install the indexes but All Sky plate solver has a settings helper that involves plate solving an image online to get an arc seconds per pixel figure and using the same image in the helper, will work out your effective focal length. Depending on the camera and software used to take it you might have to tell it your pixel size, on my setup it pulls it out of the FITS file header.
When I set my reducer/corrector up as the internet suggested (Measuring from the end of the threads not the base) I found it was giving me about 0.56 reduction not 0.63. so it is useful to compare. If it came up with significantly under 1645 focal length for an image sub with the reducer in the train then the spacing is probably out.
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