I usually mix a full 9ml (6 ml of the big vial, then a whole small vial [3ml]) according to the instructions I was given. I then put 4.5ml into my neb (I'm suppose to do 4ml, but I just figured heck I'll go halfway). I wasn't told anything about throwing out the leftovers. I dunno, I just figured heck, I store the vial where I store my neb and flutter valve, I'm not real worried about it. My brother throws the leftovers away. The deal is that I can ONLY fill this perscription where I have my CF appointments every three months, in Nashville two hours away. So I get a full 3 month supply and that's all I have to go off of.
I've started mixing 3ml of the big vial and 1.5ml of the small, I figure it would be harder for germs to get into the vial than into the tube.