I have just left it in the pre factory programed reactivity settings myself. I have hunted with GMpower at 2 and Deus Fast at 3 both factory settings. Both programs are virtually preset identicaly except for this and the preset frequency if I remember right.
Only thing I changed was the frequency to 12khz from 18khz more for general all around coin hunting. 18khz supposed to be for very mineralized ironized hot rock soil and beach/sand areas as it's more sensitive it states in the manual.
12khz for mineralized hot rock soils all around hunting it says in manual.
8khz for hunting in low mineralized soils for coins and larger masses it says.
4khz is for large masses and coins in fairly non mineralized soils or plowed field type soils and a couple of the settings are NON adjustable in this frequency and preset at the factory and can't be changed.