Hi,
Not sure if that is what you’re searching for but the rule I know is the following:
Aircraft must not be vectored closer than a half of the separation minimum (i.e. closer than 2.5 NM if the separation minimum is 5 NM) from the limit of the airspace which the controller is responsible for, unless
otherwise specified in local arrangements. [SOURCE]
Maybe someone else can add the exact doc or corrections.
BRGDS