Rather than rely on some formulaic method of agitation, one should probably rely on the actual contents of the negative to determine agitation. With decreased agitation, separation between high and low values is decreased. The opposite is true for increased agitation. With higher values, the developer exhausts quicker than with lower values, therefore your development and agitation should be matched to some context of the individual scenes. By altering your agitation, as with altering your times, you change the range of values. The greater the alteration, the greater the change. As agitation is only part of it, I should think there is no magic one size fits all.
I have found that changing agitation to match my scenes gives me better pictures than using a solid set in stone standard.