I use this combination and go for the increased Dmax rather than eggplant/red color shift also.
I mix KRST with a regular strength Permawash solution. I usually tone in batches so use the toner on a one-shot basis. I dilute KRST 1:15 in the Permawash solution for about 2:45. I say "about" because I always test the first print in the batch by itself first to make sure that I am not overdoing it. I try to leave it in the toner just long enough that it does not flip to purple. If you increase the strength of the toner to say 1:10, that is much harder to control and the toning times become very short. If you increase dilution beyond 1:20, the times get longer and the effect seems more subtle.
My dilution of 1:15 is not really exact in that I add 125 ml of KRST to 2000 ml of Permawash to keep the measurements simple. So I am truly using about 1:17.