$subject says it all, this is mostly just a tracker bug so that we can have dependent bugs.
synaptics behaviour is to use a Schmitt trigger with a default of 9.8% for 'low' and 11.7% for 'high' (25/256 and 30/256, respectively). There are model-specific cases, (some) apple touchpads use an absolute value range of 70/75 and all elantech touchpads just use the pressure minimum + 1.
The elantech value is questionable though and should be reviewed.
libinput likely needs some extra bits to be able to assign per-device values from the udev hwdb or similar.
Hi I have been brought here by https://bugs.kde.org/show_bug.cgi?id=375406#c2
You may be interested in it, even if it is xorg related.
Have a nice day
Merge: b9b0f36 9e8f6bd
Author: Peter Hutterer <email@example.com>
Date: Fri Feb 3 11:41:04 2017 +1000
Merge branch 'touchpad-pressure-based-v2'