A bit searching reveals this Reddit post: https://www.reddit.com/r/archlinux/comments/6ekl94/x220t_and_wacom/dib6928/?utm_source=reddit&utm_medium=web2x&context=3
where you could run
xinput_calibrator --device <Device ID>
(With the correct Device ID of course).
And then, set the correct values from the output above with :
xsetwacom set "<Device Name>" Area <topX> <topY> <bottomX> <bottomY>
The xinput_calibrator can be installed from the AUR: AUR (en) - xinput_calibrator
The ArchWiki has big entry about Wacom Tablets but it seems nothing about calibration.