Till it's filled appropriately with the res near the top line... :O
I had no issue buying a standard jug of 50/50 mix, and had some left. As for flushing... Distilled water is like $0.99/gallon and... I'll drink any that doesn't go into the bike, so I think I got 5 gallons last time drank one while doing the procedure and flushed it "quite well".
When I did water wetter one bottle did the trick.
I'm not really precise about it, because some comes out during the burping process.