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Close but no cigar.
Longest eyepiece focal length for your scope = telescope focal ratio x 6mm
Mid-power eyepiece focal length = telescope focal ratio x 2mm
Shortest useful focal ratio for your scope on most "good nights" = telescope focal ratio x 1 mm assuming the optics are good.
Absolute shortest you could ever use = telescope focal ratio x 0.7 mm, but only if the telescope is a really excellent optically (many aren't).
The latter might be useful about 1 night per year.
IMHO you could get by with just three eyepieces - low, medium and high power where medium is the geometric mean (not arithmetic mean) of low and high.
Alternatively 4 eyepieces, the focal lengths increasing at roughly at 1.5x from one to the next.
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