Keep in mind that Page_Event "12" is an "Exit Link"... which is only recorded when someone clicks on a link in the site that goes to a domain that is not listed as external (and this will only trigger if Exit Link Tracking is enabled).
However, if the "exit rate" definition for @fv394021 is actually based on the "last page of a visit" (i.e. they want to know what the rate of exits are on Page X (including the user closing the browser, or walking away and letting the session expire, etc), then page_event 12 won't help...
This you will have to identify the visitor, and the visit number for each visit, then identify the last page view of the visit (highest visit_page_num, or I suppose you could do it based on timestamps).. Make sure to only look at Page Views (exclude highest visit_page_num that are actions), then you can calculate:
"last page views on Page X"
/
"all page views on Page X"
This will now not be dependent on people clicking on exit links...
But this all depends on your definition of "Exit Rate"