Hi,
$WINDOWID might be what you're looking for.
Note that whatever you do, it'll probably be an ugly hack that just
happens to work with your favorite terminal at a hardcoded font size.
E.g. how would your X app figure out the terminal's font size?
Also, keep in mind that there are terminals that have multiple tabs and
your one might not be the visible one (e.g. gnome-terminal -- also
beware that it's known in gnome-terminal that if you drag-n-drop a tab
to another gnome-terminal window then $WINDOWID becomes incorrect
because it can't be updated); there are terminals with split screen
feature (e.g. gnome terminator) and then how do you figure out the
offset; etc...
e.