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A 7" f/10 mirror has no need to be parabolised, it may as well be spherical.
As for f/13, not suitable for a Newtonian - buy an f/15 maksutov.
I appreciate your comment about eyepieces but IMHO the optimum is f7 - the maximum useable exit pupil is achieved with a 42mm eyepiece (lowest power) and the maximum useful magnification is achieved with 4 or 5mm eyepiece there are many to choose from as f/7 motors are not hard to make (but will need to be parabolised).
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