I use Tamiya paints and keep the empty bottles if I have to mix a color I know I will use again I mix it and put it in an old bottle and label it. You are not going to avoid losing paint when you mix paint. When I mix and keep it like that I use a shot glass with measurements on it for mixing in those amounts. Tamiya is big on giving you mixes for some colors  and if it's just a one time mix, I mix it in the airbrush using a stir spoon to add the appropriate drops the instructions call for. If the shade isn't perfect it doesn't really bother me. If hand painting I mix it with retarder in one of those multiple cup plastic trays. Like I said though when mixing you will lose paint and I stopped worrying about that some time ago. It's just a fact of paint mixing life.