You might want to submit this to the YamahaSynth IdeaScale.

Just to clarify.

“Pitch Key Follow Sensitivity“ has a range from 0-99 which is, more or less, a minimum-to-maximum range where at one end behavior is one way, and at the other you arrive at the other possible behavior. In FM-X it can be used to add non-Equal Temperament change to an Operator that is set to Fixed Frequency Mode — a setting of 0 causes that Operator to output a single fixed frequency across the keys, as you reach 99, that would be virtually 100% toward behaving exactly like an Operator set to Ratio Mode (variable pitch where Equal Temperament is the default).

Understanding the 0-99 as Fixed Frequency - to - Ratio behavior.

In Dr. Manny Fernandez’ tutorial on his FM-X acoustic piano —“Acoustic Eccentricities and Stuff”—, he discusses using Fixed Frequency Modulators and their use in creating the very inharmonicities you mention. And how he does this with several Operators to fill in the acoustic behaviors. It is the fixed frequency components which are, indeed, important in emulation of musical sounds. It is they that “give it all away” when pitch shifting an audio sample. When the ear/brain hears what it knows does not pitch shift, change, it becomes unusable. The fixed frequencies in your speaking voice, if playback speed is altered even slightly, this will cause your friends to laugh. They’ll know it’s you, but the fixed frequency components now ‘give it away’. We don’t really notice them until they’re missing or not right.

A plink (metallic sound) or thunk (wooden sound) that is inherent in acoustic pianos, do add to our acceptance of what we are hearing when attempting to synthesize the sound. If it changes pitch (as in samples) too severely the ear identifies that immediately. But fixed frequencies are not always absolutely fixed either, they may change at a different rate.

As you approach 99 you approach normal Ratio behavior (whole integer relationships are ideal for creating harmonic behavior).
As you reach 0 you are fixing the result across all keys... every where in between is some degree of (non-equal) pitch change

What you are asking for, (perhaps) is greater resolution between Fixed and Ratio behavior.
Manny also uses Pitch/Vel sensitivity to add to the change in character of this component.
And uses additional Fixed Frequency Operators to fill in the holes...

As you know, with greater and greater control, you can do most anything with FM. You may be able to accomplish your goal by using additional Operators to “fill in the harmonic holes”, as necessary... what he calls “Stuff tricks”...

In your experiments, where, along that minimum-to-maximum scale, do you feel more resolution is necessary?
Just curious...

Hi Bad Mister, I've talked to Manny, but an increase in resolution is not what I need. I need the parameter to go beyond equal temperament, to 120% or more to emulate the Yamaha VP1 patches. In the upper keyboard ranges the Piano harmonics are about 105% spacing, not as dramatic as the VP1, but greater than equal temperament. In FMX I can use a modulator to create the harmonic fans at any ratio, but I need the harmonic fans to change as you move up the keyboard. The spacing is 101% down around C2, and greater than 105% up at C5. Since Pitch Key Follow Sensitivity only ranges from 0 to 100% of equal temperament, I can't reach 105%. In all string instruments the Inharmonisity shifts the harmonics Sharper, and all I can do now is shift them Flatter.

Does that explanation make more sense?
It should be pretty easy to add a few percent on the top of the Pitch Key Follow Sensitivity range in the firmware.

Yes it does.

Currently, there is a “Pitch Key Follow Sensitivity” which can be set -200% ~ +200% but this is a Common setting and applies to the overal FM-X Tone...

Page 134:
Pitch/Key (Pitch Key Follow Sensitivity)
Determines the sensitivity of the Key Follow effect (the pitch interval of adjacent notes), assuming the pitch of the Center Key as standard.
Settings: -200% – +0% – +200%
+100% (the normal setting): Adjacent notes are pitched one semitone apart. 0%: All notes have the same pitch as the Center Key.
Negative values: The settings are reversed.

As I’m understanding this then... your request is to be able to have this Control per Operator... specifically, to be able to apply an extended range of frequency change on Modulators that are Fixed Frequency. Is that correct?
The (current) 0-99 setting applied to an individual FIXED Frequency Operator, determines the change in pitch between it and the adjacent key, as in 0 = no change, and 99 would mean the adjacent note is a semitone away from its neighbor. Thus a FIXED Frequency Operator set to 99, behaves like an Equal Temperament tuned Operator. (Like when Common Pitch Key Follow Sensitivity = +100%)

Page 143:
Pitch/Key (Pitch Key Follow Sensitivity)
Determines the sensitivity of the Key Follow effect (the degree depending on their position or octave range). This is available only when “Freq Mode” is set to “Fixed.”
Settings: 0 – 99
0: All notes are the same pitch specified by Coarse and Fine.
99: Adjacent notes are pitched one semitone apart.

You’re looking to have extended the pitch range control on the Fixed Frequency Operator (Modulator)... just wanted to clarify because (well, I wasn’t clear). I understand your request better, now...

Yes sir, that's exactly what I am requesting. I want to be able to use a fixed frequency modulator, and set the Key Follow Sensitivity to greater than 99. I want to shift the FM harmonics sharper as you go up the keyboard so I can match the shift I've measured in Pianos, Mandolins, Violins, and other stringed instruments. The Yamaha VP1 patches also do this and are my second target.

I would also request an Inharmonicity feature, but that isn't as likely to be implimented. Increasing the range of the Key Follow Sensitivity is simple, I'm really hoping this will get implemented.

Thanks for your help!