It's not possible for a human to push down on the tib of the pen with a perfect monotonically changing amount of pressure.
To do this I use a Kuuube's pressure emulation plugin for OpenTabletDriver.
You can download the plugin from here: https://github.com/Kuuuube/Pressure_Emulation
This is my standard configuration
Pressure Resolution - to simulate N levels of pressure. Shown below is 256. Setting it to zero will not affect the pressure values
Line length - 800
Line X Offset - 200