The Locator Extra system has an extended face which allows the user to install the rail in the corner of a square-set ceiling (no cornice). To get the perfect height of the fixings so that the rail touches the ceiling, the fixings should be installed to between 17-18mm centres from the ceiling line. The installer can of course use a template such as a piece of card for this, or of course a small piece of the rail turned over with a centre-line marked in the fixing cavity.
It is always a good idea to do a test piece before installing all of the fixings as it can vary a bit depending on the method used by the builder in setting the corners of the ceiling. The rail can be sealed after the installation is complete to conceal the small gap completely.