Should be set to 25% of main alliances power total. That way it is decent alliance but not too powerful.
Also allow a 10% tolerance within that power loss from the main alliance to adjust the sub alliance within 24 hours or disbandment will occur. So the sub alliance could be 35% of the total power of the main alliance but has 24 hours to either lose the 10% of power or have the main alliance gain 10% total power to get it out of the red zone for disbandment.
But that is my opinion though.
DARK