1+2 or 1+3 is usual. Do them in strips in a white tray -- that way you can see the intensification. Generally, one tones them until one sees nothing else happening -- it is hard to "over-do it" as the toning continues until there is no more silver available for the selenium to act on. One can get about a grade of contrast more, depending on the film and its exposure/development. Toning will not put information on the neg where there is none (due to under-exposure of the film).