What do you mean by "optimum" toning? Simply DMax with selenium? Because toners are also used for, well, "tone", actually meaning hue in this case. And when used more than one at a time, both the perceived and measured DMax are affected. So guess you'll have to narrow your parameters to a specific paper and developer (and specific dev time, temp, and concentration) if you want specific results. Even the color mix on VC papers is a factor.