All defaults are invented. Step 1 turns a four-pin Wenner reading into soil resistivity (2 x pi x spacing x reading). Step 2 estimates one vertical rod with the Dwight formula in uniform soil. The result is a model, not a measurement. Measure the installed electrode.
The 25 ohm target is a user input. Your AHJ and adopted code decide what applies. The model assumes one vertical rod, top level with the surface, in soil of one resistivity. Real soil has layers and changes with moisture and frost.