I recently thought about this in the context of FOC-Stim.
FOC-Stim being a device with absolute volume controls is really nice, you can experimentally determine the volume which causes you to climax with decent accuracy, it seems to be about 10% range between sessions (for science

). At the start of every session you set the volume to that level and then you climax when the funscript approaches 100% quite reliably.
When the volume decreases, the current in FOC-Stim decreases linearly. I start to feel something at like 10%. I don't remember the exact figure, but there is about a factor 10 difference between the detection threshold and pain/cum threshold.
It's not so nice with a stereostim box. The box controls the voltage and there are many factors (winding ratio, voltage, electrode material, electrode size, moisture) that determine the current. So it's possible you don't feel anything at 50%. Everyone that tried the FOC box seems to agree the volume range is less compressed with FOC (meaning increasing from 80% to 90% results in a much bigger increase in signal strength on stereostim than on FOC). This is particularly noticeable with mistress and the remote control box. The 'good boy' sections don't pack much of a punch on FOC.
There are two ways to fix this. Option 1 is to keep FOC-Stim as-is and define volume as linear with current. This makes it exceptionally easy to program future boxes (neoDK, noisy cricket..) with identical volume ramps. The disadvantage is that this breaks all existing funscripts since they were made for the nonlinear response of stereostim boxes as Edger477 points out.
The second approach is to apply some sort of calibration adjustment to FOC-Stim so it has the same wacky volume-intensity curve as stereostim (or more accurately, one particular stereostim box which I characterized at one particular frequency). Initially this sounded like a good idea because it allows us to re-use all existing .volume.funscript, but I'm not really a fan. It seems nearly impossible to agree on a standardized curve because it depends on so many factors.
If you want to make volume funscripts today, I recommend using the full 0-100% range in restim and take Edger477's advice. I don't want to create a situation where every funscript requires different restim settings, that would be annoying.