Depends on the paper, time in the toner, the age and exhaustion of the toner, etc., etc. In other words, there is no hard and fast relationship between toner dilution and change in image tone.
Experiment; change times, dilutions, papers, etc. till you find what you like. It is largely a visual process. Keep notes so you know what papers react how to weaker or stronger toner. Watch out for split toning (some papers tone faster in the shadows than the highlights).
FYI: Selenium toner adds red/purple to the image tone. At high dilutions and/or short times, it neutralizes the greenish cast many papers have, and moves the image tone first to a more neutral black. It will not "cool" the tone of a print at all. If you want cooler colors (i.e., more blue-blacks, try different developers, benzotriazole and/or gold toning.
And yes, I am am speaking from
experience. I tweak the dilution of my selenium toner by adding small amounts of toner or water to get the time/tone I am after. It is only predictable if all of the parameters are the same (paper, batch, print developer and time, etc., etc.,). The process is rendered practically unpredictable by the fact that each print run through a toning solution removes some of the toner and makes it weaker. I know of no way to accurately replenish, since every print is a bit different. The end result is that toning times lengthen and the aggressiveness of the tonal shift falls off with continued use. I replenish by
feel, adding a bit of stock toner solution to the working solution when I
feel that the times are too long or I'm not getting the tone I want.
BTW, don't toss your toning solution. Filter it, save it, replenish it and use it forever. Don't burden the environment with heavy metals!
Have fun,
Doremus Scudder
www.DoremusScudder.com