It should be added, the reason for changing feedline length is to bring its
input impedances on the various bands into ballparks which can be
accommodated by the tuner. Ideally, input impedances should not be VERY
much different from 50 ohms. SWR not much worse than 1.5 or 2-to-1.

The line behaves as a different impedance transformer on every band. It is
a random adjusting process.

In principle, with a perfect tuner, ANY line length would be satisfactory.
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Reg, G4FGQ