Small(kinda?) configurable shell script to calculate active tablet area units for Xorg drivers (You'll have to adapt the output depending on which driver you're using)

!tabletarea.sh

#!/bin/sh

#Monitor display ratio
disprat=$(echo "16 / 9" | bc -l)

#Full area size in real units (for the G640, in mm)
tabw=152.4
tabh=101.6

#Full area size in driver units
driw=32768
drih=32768

#Execution:

echo -n "Enter Top X (0): "
read x0

echo -n "Enter Top Y (0): "
read y0

echo -n "Enter Width (/$tabw): "
read w

[ -z $x0 ] && x0=0
[ -z $y0 ] && y0=0

[ -z $w ] && w=$tabw

h=$(echo "$w / $disprat" | bc -l)

# Turn them into ratios
w=$(echo "$w / $tabw" | bc -l)
h=$(echo "$h / $tabh" | bc -l)

# Add the offset and scale for the drivers
x1=$(echo "(($x0 + $w) * $driw) / 1" | bc)
y1=$(echo "(($y0 + $h) * $drih) / 1" | bc)

msg=$(cat <<'EOT'
Edit your Xorg tablet configuration file with these options:
    Option "TopX" "%d"
    Option "TopY" "%d"
    Option "BottomX" "%d"
    Option "BottomY" "%d"
EOT
)

printf "$msg\n" $x0 $y0 $x1 $y1

52-tablet.conf

# Example configuration file for an XP-PEN STAR G640 tablet, full area mapped to a 16:9 monitor.

Section "InputClass"
    Identifier "Wizardpen Tablet"
    MatchIsTablet "on"
    MatchProduct "XP-PEN STAR G640"  # Change this to your tablet model.
    Driver "wizardpen"
    # Edit these lines to your wanted area.
    Option "TopX" "0"
    Option "TopY" "0"
    Option "BottomX" "32359"
    Option "BottomY" "27303"
    # Apply custom Options below.
EndSection