I did a search for the CP 80 manual, and am only finding it behind paywalls (average cost around $16) or from sites that are attempting to give me trojans. Yamaha's only CP series manual, if I did the search correctly at their site is for CP 88 (or a supplement with additional voices that came out later). Searches on the words "pitch" "intonation" "tuning" didn't show what I was hoping to find.
If the chart is a true Railsback implementation, I'd seriously consider purchasing it. But I can't tell from the info you gave if that is indeed what it is. You mention a pitch range (-23 to +35 cents). This suggests to me that the setting is used for coding temperaments. For the "Railsback curve" stretch, each octave, working outward from Middle C, would have more deviation than the previous, and so, the desired amount of deviation would lie outside the range.
The nice thing about the DX7 series performance intonation settings is that as the stretch accumulates, it is possible to make use of the "coarse" control, if needed.
