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An interesting snippet of info that may help you is to look down the open end of the telescope at the primary and then start walking backwards. At one point the image will flip. This point will be your focal length of the primary (measured from where you were when the image in the primary flipped, to the primary.
Armed with that info, you can measure the distance from the primary to the secondary, and the secondary to the eyepiece. These two distances added should be the same as the focal length you just measured by eyeballing the primary.
Dean
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