Assume that the focal length stated for the scope is at the point where a
25mm EP attached to the back of the scope in the normal position. Any
length you add by moving the primary relative to the secondary will add to
that focal length.
Any changes will be relatively minor when you calculate the magnification so
why worry about it? For precise accuracy, you will still have to measure
the image scale in a photograph to know what the image scale is.

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Why do penguins walk so far to get to their nesting grounds?