A simply supported prismatic concrete beam of rectangular cross-section, having a span of $8\:m$, is prestressed with an effective prestressing force of $600\:kN$. The eccentricity of the prestressing tendon is zero at supports and varies linearly to a value of $e$ at the mid-span. In order to balance an external concentrated load of $12\:kN$ applied at the mid-span, the required value of e (in $mm$, round off to the nearest integer) of the tendon, is ______.