Mixing paint for airbrushes??

I was just wondering how ppl mix piants for airbrushes cause you have to mix piant with thinner right?
I use Tamiya paint and everytiem i mix i almost mix the whole bottle of the paint. Cause like lots of the paint stick to the mixing bottle and are wasted. Also if i'm just doign one small part liek the exhaust how do i mix the paint cause so little is needed.

i use the tamiya paints acryl i use metholayted spirits and the enamels i use turpintine. i mix around 40-50% and if im doing a small area like a exhaust i add the paint and thinner to the paint cup and mix.when im finished i pour back into the paint jar. Sometimes i keep the mixed paint in a baby food jar for the next time