Ops manual - 35% normal limit, up to 40% allowed.
But there are places where 40% will not be enough. There you do whatever is necessary. Mexico City - 757 wouldn't move with 50% N1. Called MX to a/c - "no problem." Eventually took about 70% to move from the rut in the taxiway. They disconnect the planes from the tug at the same spot and the ruts keep getting bigger and bigger.