Follow the path of the code, remember this is just so you can get the past point.
;Define the variable
(if (= last_point nil)(setq last_point (list -1 -1)))
;Call the dialog
(if (not (new_dialog "your_dcl" dcl_id "" last_point)) (exit))
;Set the last point
(action_tile "ok" "(do_what_ever)(setq last_point (done_dialog 1))")