First of all, thank you very much Scawen for considering my suggestion, and also big thanks for asking us for our thoughts about it.
As someone who developed an FFB wheel firmware and also my own GUI, where I had to deal with exactly these questions how to make it transparent for the user and how to make it simple and intuitive to use.
It seems that working in % of axis range is best for setting calibration ranges. So instead of having +-1000 I would go with +-100%, with a step of 1% adjustment, which should be ok.
About axis sensitivity (non-linearity), I would leave this out and simply keep a linear axis between the user-defined min and max limits in %. If you really want you can add a 3rd point - center, but not really necessary at all. Can be done with a 3rd party software like DxTweak.
In principle, whether you choose +-1, +-100% or +-1000 I don't mind, as long as in this range there are as many steps as controller reports by HID.
It would be good to see somewhere the raw axis value as reported by DirectInput. It should fit into graphics in FF/axes, I got used to it and it is simple and effective, so I don't think any changes are needed there. So instead of showing a momentary value of the axis between -1000 to 1000, I would put there a raw HID value for that axis, like 0-65535, or 0-4095, 0-1023, 0-255. With this, a user can directly see the resolution of his controller for each axis and at the same time an axis value.