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Conrady's rule is a good start but ... I've used two excellent refractors - 4.25" f/16 and 9" f/18 that were made long before Conrady which suggest that rule is overly conservative with good design.
Wiith modern glass types a doublet could achieve a focal ratio 2x the aperture in inches. BK7 + F2 is a very lazy solution.
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