I'd say that there is another requirement - range used for SOL calibration and range used for measuring should be identical! More precisely, calibration points and measuring points should be the same. This is less impactful if measuring range is relatively small (narrow) comparing to the "center frequency" (for example 14.000MHz to 14.300MHZ vs 3.5MHz to 29MHz), but still ...
With lengths approaching quarter wave length, things (impedance transformation) become very fast changing, and in my experience, interpolation between points where actual calibration was done done becomes much less accurate.
