Just read through this DIY.
Brake system holds about 1 liter. That would mean full pads, full reservoir. Remember as your pads wear, the level goes down. If your pads are low, don't add fluid or you'll push it out when you attempt to replace your pads. It's a semi-sealed system.
An alternative to sucking out all of the old fluid from the reservoir is to do one/two dumps down to MIN line at the front left caliper. That way crap doesn't have to get pushed to the back. The front left is a REALLY short run so be ready for a quick flush. Then just start the process and redo the front left.
Also...flush does not mean you should drain your fluid and add more. You never want to empty your system unless you have a flat bed to the dealer lined up. If you know I'm wrong, I'm not talking to you.