Ken, I guess I'm still confused.

As I understand it, one cannot reliably determine the exact resonance of
a dipole by finding the point of minimum SWR. Until this measurement
issue is resolved, there would seem to be little benefit to seeking an
explanation of why the formula appeared not to work.

It is probably too late now, but if you had used an impedance bridge
(MFJ or Autek, for example) you could have found resonance at the point
of zero reactance. All within the limits of the instruments, of course.

Some of the posts suggest other reasons why the formula might not work,
but it is not yet evident to me that it didn't work.

Sorry my earlier post was not more clear. (I'm even sorrier for this
post if you actually used an impedance bridge! Hi.)

73,

Chuck
NT3G