I never really trust that I've made a print of the "optimum" contrast and exposure until I make ones that I don't like at all extremes (too much/too little contrast and too much/too little exposure). Yes, it's sometimes a waste of a sheet of paper just to make sure I was right (I'm not gonna like this with more contrast, but let's try it anyway...), but sometimes it works out (hey... I kinda like this with really open shadows!).
And, of course, there is an area of acceptability as well: a range of contrast and exposure that I find pleasing, leaning one way or the other depending on mood, what lighting I'm displaying under, etc. Kind of like tempo and dynamic variations in a musical performance: many valid interpretations possible.
Anyway, I find I can save time by making rather large differences in the beginning of the refinement process, so I can get those extremes I don't like out of the way right away, or, conversely, not waste time refining a print in one way when I really like it another. I never jump less than a paper grade, sometimes I jump 2 (or make an analogous change with print developer/brand). The same with exposure. 20% is really not enough of a difference to get to an extreme quickly; I like to start with at least 40% more or less. (As you can gather, I prefer using percentages to f-stop timing or purely arithmetical changes).
Once I've gone to far, it's fairly easy to estimate an intermediate point to start the real refining from.