We can see from the data sheet on Ilford Multigrade Paper (link) that the curve gets steeper with increased contrast and tonal range shrinks (as expected). Various techniques seek to maximize tonality, and I'm a fan of the split grade printing technique. Let's assume I've printed an image with the maximum tonality that I can achieve (given my artistic contrast needs, of course :-)
Now I'm going to split tone it in sepia. In the past, I've done this by trial and error to get something that I like. What my question is: is there any more scientific way of going about toning? What happens to the print's characteristic curve during toning? Can the coloration be quantified and graphed as a product of the original characteristic curve and some modifier for time in bleach and/or chemical dilution?
Just as I can look at a film characteristic curve and understand how much latitude I have to work with and (pending development) how much contrast as well...how can I have this same insight into toning?