the RB cooling system is 8L so 2.5L is a little low...
how did you flush it? are you sure you got all the coolant out?
the radiator itself probably takes around 2.5L, maybe you didnt get all the coolant out of the runners
when bleeding, run from cold with the heater on until the thermostat opens then recap it after the bubbles stop. take it for a drive, let it cool down and top it up. cooling systems are biatches! ive just gotten over all my problems, a pin-hole leak in one of the hoses caused a blown radiator and water pump - nice