There is no height sweet spot, unless you operate one band only. Good dxing height is related to wavelength and topography. A 60' antenna on 15m may work fine with good takeoff topography, but its definitely too low for a competitive shot of 10k-12k miles on 40m. Very competitive dxers sometimes use multiple heights (per band) to excellent advantage. "Sweet-spot" has some meaning considering bang for the budget, but doesn't well accommodate the needs of an electromagnetic field. The best rule-of-thumb I ever heard fifty years ago, when new to dxing, was "use at least one wavelength high if possible." Since then I've used heights from 0.2 wavelength indoor height to 1.7 wavelength outdoors, and several heights in between. The effects of height were astounding at times.