I guess I'm lucky, I've done the coolant a few times on mine now, the last being a full change and flush last week.
I always remove the top bleed screw whilst filling and fill it with a little watering can and measure how much i put in.
once the level has filled to the top of the expansion tank I squeeze and pulse the hoses and the level drops in the expansion tank. I then fill it up again and start the motor. usually the level drops and then I start getting water out of the top hose bleed. I keep topping it up with the engine running till it comes out like a little fountain and let it run till the level in the tank drop from full to normal. Put the screw back in and its usually bang on and doesn't move. I watch it like a hawk for a few weeks just incase though