There are probably as many ways of toning, as there are photographers on this site. I can only relate what I have found works for me over the years.
Originally I diluted the KRST with HCA 1:9. This worked well but the solution life was short due to the oxidation of the HCA. This is handy as a one-shot but not as a reasonably stable reusable lot. Recently I have mixed the KRST 1:8 with distilled water only. This solution lasts a longer time.
My technique is to fix the prints in two solutions (the two-solution fixing procedure – too much to go into here), rinse under flowing water, HCA, 2 minutes RC, 4 minutes fiber, rinse in flowing water then into the KRST.
At this point things get interesting. I still don’t understand the magic, but my prints loose the greenish cast in about two minutes and then, depending on the brand and type of paper, get a nice three-dimensional quality that I really like. This comes form a professional chemical engineer who should know what is going on – I don’t. The selenium forms several complexes with the silver in the print. By reference to the reactions of silver with sulfur, I presume the actual reaction products are quite complex and frankly, beautiful, and that is sufficient for me.
Kodak Polymax RCII does not seem to get the color, but the green disappears and the contrast is increased slightly (Intensifies?) Other papers react differently, some gaining a purplish cast. In my opinion, one of the loveliest renditions is achieved on Ilford Galerie.
One must experiment a little to observe the results on their choice of materials. The bottom line is that selenium toning aids in print longevity, especially RC prints, and alters the appearance in a manner chosen by the observer with their materials time in the toner.